diff --git a/src/assessments/assessment-default-message-generator.tsx b/src/assessments/assessment-default-message-generator.tsx
index 0b89c88358d..ad90f978474 100644
--- a/src/assessments/assessment-default-message-generator.tsx
+++ b/src/assessments/assessment-default-message-generator.tsx
@@ -17,10 +17,10 @@ export type IMessageGenerator = (
 export type IGetMessageGenerator = (
     generator: AssessmentDefaultMessageGenerator,
 ) => IMessageGenerator;
-export interface DefaultMessageInterface {
+export type DefaultMessageInterface = {
     message: JSX.Element;
     instanceCount: number;
-}
+} | null;
 
 function failingInstances(result: TestStepResult): boolean {
     return result.status !== ManualTestStatus.PASS;
diff --git a/src/common/components/flagged-component.tsx b/src/common/components/flagged-component.tsx
index 08110cc8c3e..031b5adcdbe 100644
--- a/src/common/components/flagged-component.tsx
+++ b/src/common/components/flagged-component.tsx
@@ -12,7 +12,7 @@ export interface FlaggedComponentProps {
 }
 
 export class FlaggedComponent extends React.Component<FlaggedComponentProps> {
-    public render(): JSX.Element {
+    public render(): JSX.Element | null {
         const flagName = this.props.featureFlag;
 
         if (this.props.featureFlagStoreData != null && this.props.featureFlagStoreData[flagName]) {
diff --git a/src/common/components/with-store-subscription.tsx b/src/common/components/with-store-subscription.tsx
index e508ff19fc8..033affaa4fe 100644
--- a/src/common/components/with-store-subscription.tsx
+++ b/src/common/components/with-store-subscription.tsx
@@ -26,7 +26,7 @@ export function withStoreSubscription<P extends WithStoreSubscriptionProps<S>, S
         constructor(props: P) {
             super(props);
             if (this.hasStores()) {
-                this.state = this.props.deps.storesHub.getAllStoreData();
+                this.state = this.props.deps.storesHub.getAllStoreData()!;
             } else {
                 this.state = {} as S;
             }
diff --git a/src/common/store-proxy.ts b/src/common/store-proxy.ts
index 333ddc43856..50ca09faa6f 100644
--- a/src/common/store-proxy.ts
+++ b/src/common/store-proxy.ts
@@ -12,10 +12,10 @@ import { StoreUpdateMessage } from './types/store-update-message';
 export class StoreProxy<TState> extends Store implements BaseStore<TState> {
     private state: TState;
     private storeId: string;
-    private tabId: number;
+    private tabId?: number;
     private browserAdapter: BrowserAdapter;
 
-    constructor(storeId: string, browserAdapter: BrowserAdapter, tabId: number = null) {
+    constructor(storeId: string, browserAdapter: BrowserAdapter, tabId?: number) {
         super();
         this.storeId = storeId;
         this.browserAdapter = browserAdapter;
diff --git a/src/common/stores/base-client-stores-hub.ts b/src/common/stores/base-client-stores-hub.ts
index 641d5a6332b..5ab850718f8 100644
--- a/src/common/stores/base-client-stores-hub.ts
+++ b/src/common/stores/base-client-stores-hub.ts
@@ -46,7 +46,7 @@ export class BaseClientStoresHub<T> implements ClientStoresHub<T> {
         });
     }
 
-    public getAllStoreData(): T {
+    public getAllStoreData(): T | null {
         if (!this.hasStores()) {
             return null;
         }
diff --git a/src/common/stores/client-stores-hub.ts b/src/common/stores/client-stores-hub.ts
index 0e2e5fc3226..fdc9ab4a692 100644
--- a/src/common/stores/client-stores-hub.ts
+++ b/src/common/stores/client-stores-hub.ts
@@ -8,5 +8,5 @@ export interface ClientStoresHub<T> {
     removeChangedListenerFromAllStores(listener: () => void): void;
     hasStores(): boolean;
     hasStoreData(): boolean;
-    getAllStoreData(): T;
+    getAllStoreData(): T | null;
 }
diff --git a/src/tests/end-to-end/common/browser.ts b/src/tests/end-to-end/common/browser.ts
index cc6731db47b..486621ce004 100644
--- a/src/tests/end-to-end/common/browser.ts
+++ b/src/tests/end-to-end/common/browser.ts
@@ -136,7 +136,7 @@ export class Browser {
                     resolve(tabs[0].id),
                 );
             });
-        });
+        }, null);
     }
 
     private async waitForBackgroundPageMatching(
diff --git a/src/tests/end-to-end/common/page-controllers/page.ts b/src/tests/end-to-end/common/page-controllers/page.ts
index 128388498b5..b93c0444bdf 100644
--- a/src/tests/end-to-end/common/page-controllers/page.ts
+++ b/src/tests/end-to-end/common/page-controllers/page.ts
@@ -50,11 +50,12 @@ export class Page {
             forceEventFailure(`'pageerror' (console.error): ${serializeError(error)}`);
         });
         underlyingPage.on('requestfailed', request => {
+            const failure = request.failure()!;
             const url = request.url();
             // Checking for 'fonts' and 'icons' in url as a workaround for #923
             if (!includes(url, 'fonts') && !includes(url, 'icons')) {
                 forceEventFailure(
-                    `'requestfailed' from '${url}' with errorText: ${request.failure().errorText}`,
+                    `'requestfailed' from '${url}' with errorText: ${failure.errorText}`,
                 );
             }
         });
@@ -100,7 +101,7 @@ export class Page {
         await this.screenshotOnError(async () => await this.underlyingPage.close());
     }
 
-    public async evaluate<R, Arg>(fn: PageFunction<Arg, R>, arg?: Arg): Promise<R> {
+    public async evaluate<R, Arg>(fn: PageFunction<Arg, R>, arg: Arg): Promise<R> {
         const timeout = DEFAULT_PAGE_ELEMENT_WAIT_TIMEOUT_MS;
         // We don't wrap this in screenshotOnError because Playwright serializes evaluate() and
         // screenshot() such that screenshot() will always time out if evaluate is still running.
@@ -111,7 +112,7 @@ export class Page {
     public async waitForSelector(
         selector: string,
         options?: WaitForSelectorOptions,
-    ): Promise<Playwright.ElementHandle<Element>> {
+    ): Promise<Playwright.ElementHandle<Element> | null> {
         return await this.screenshotOnError(
             async () =>
                 await this.underlyingPage.waitForSelector(selector, {
@@ -134,7 +135,9 @@ export class Page {
         });
     }
 
-    public async getSelectorElement(selector: string): Promise<Playwright.ElementHandle<Element>> {
+    public async getSelectorElement(
+        selector: string,
+    ): Promise<Playwright.ElementHandle<Element> | null> {
         return await this.screenshotOnError(async () => {
             return await this.underlyingPage.$(selector);
         });
@@ -167,12 +170,17 @@ export class Page {
         selector: string,
         options?: WaitForSelectorOptions,
     ): Promise<string> {
+        if (options?.state === 'hidden' || options?.state === 'detached') {
+            throw new Error("Cannot get properties about selectors that shouldn't be there");
+        }
         return await this.screenshotOnError(async () => {
             const element = await this.underlyingPage.waitForSelector(selector, {
                 timeout: DEFAULT_PAGE_ELEMENT_WAIT_TIMEOUT_MS,
                 ...options,
             });
-            return await this.underlyingPage.evaluate(el => el.outerHTML, element);
+            // element! is safe because we verified that options.state is not one of the options
+            // that can result in waitForSelector returning null
+            return await this.underlyingPage.evaluate(el => el.outerHTML, element!);
         });
     }
 
diff --git a/src/tests/end-to-end/common/scan-for-accessibility-issues.ts b/src/tests/end-to-end/common/scan-for-accessibility-issues.ts
index e6dc7c5f76d..b807dcfc3be 100644
--- a/src/tests/end-to-end/common/scan-for-accessibility-issues.ts
+++ b/src/tests/end-to-end/common/scan-for-accessibility-issues.ts
@@ -30,7 +30,7 @@ export async function scanForAccessibilityIssues(
 async function injectAxeIfUndefined(page: Page): Promise<void> {
     const axeIsUndefined = await page.evaluate(() => {
         return (window as any).axe === undefined;
-    });
+    }, null);
 
     if (axeIsUndefined) {
         await page.injectScriptFile(
diff --git a/tsconfig.strictNullChecks.json b/tsconfig.strictNullChecks.json
index b8ceaf21dcc..ad6eebf70ef 100644
--- a/tsconfig.strictNullChecks.json
+++ b/tsconfig.strictNullChecks.json
@@ -29,10 +29,9 @@
         "./src/DetailsView/components/switcher-style-names.ts",
         "./src/DetailsView/components/target-page-hidden-bar.tsx",
         "./src/DetailsView/components/test-status-choice-group.tsx",
-        "./src/Devtools/inspect-handler.ts",
-        "./src/Devtools/target-page-inspector.ts",
         "./src/ad-hoc-visualizations/issues/get-notification-message.ts",
         "./src/assessments/adaptable-content/test-steps/test-step.ts",
+        "./src/assessments/assessment-default-message-generator.tsx",
         "./src/assessments/audio-video-only/test-steps/test-steps.ts",
         "./src/assessments/auto-pass-if-no-results.ts",
         "./src/assessments/color/test-steps/test-steps.tsx",
@@ -112,6 +111,7 @@
         "./src/common/components/expand-collapse-left-nav-hamburger-button.tsx",
         "./src/common/components/external-link.tsx",
         "./src/common/components/fix-instruction-processor.tsx",
+        "./src/common/components/flagged-component.tsx",
         "./src/common/components/header-icon.tsx",
         "./src/common/components/header.tsx",
         "./src/common/components/issue-filing-needs-settings-help-text.tsx",
@@ -150,8 +150,7 @@
         "./src/common/navigator-utils.ts",
         "./src/common/platform.ts",
         "./src/common/state-dispatcher.ts",
-        "./src/common/stores/client-stores-hub.ts",
-        "./src/common/stores/store-names.ts",
+        "./src/common/store-proxy.ts",
         "./src/common/tabbable-elements-helper.ts",
         "./src/common/types/deep-partial.ts",
         "./src/common/types/details-view-pivot-type.ts",
@@ -329,6 +328,9 @@
         "./src/tests/end-to-end/common/element-identifiers/target-page-selectors.ts",
         "./src/tests/end-to-end/common/force-test-failure.ts",
         "./src/tests/end-to-end/common/manifest-overide.ts",
+        "./src/tests/end-to-end/common/page-controllers/content-page.ts",
+        "./src/tests/end-to-end/common/page-controllers/page.ts",
+        "./src/tests/end-to-end/common/page-controllers/popup-page.ts",
         "./src/tests/end-to-end/common/playwright-option-types.ts",
         "./src/tests/end-to-end/common/prepare-test-result-file-path.ts",
         "./src/tests/end-to-end/common/pretty-print-axe-violations.ts",
@@ -361,6 +363,7 @@
         "./src/views/content/guidance-title.tsx"
     ],
     "include": [
+        "src/Devtools/**/*",
         "src/assessments/language/common/**/*",
         "src/background/injector/**/*",
         "src/common/action/**/*",
@@ -373,6 +376,7 @@
         "src/common/message-creators/types/**/*",
         "src/common/promises/**/*",
         "src/common/react/**/*",
+        "src/common/stores/**/*",
         "src/common/styles/**/*",
         "src/common/types/property-bag/**/*",
         "src/content/settings/**/*",