[Interactive Graph Editor - Stories] Fix broken default and controlle…

…d stories (#1463)

## Summary:
The stories are broken because of a missing null check.

Adding that here.

Issue: none

## Test plan:
`yarn start`
Check that the following stories work
- http://localhost:6006/?path=/story/perseuseditor-widgets-interactive-graph-editor--default
- http://localhost:6006/?path=/story/perseuseditor-widgets-interactive-graph-editor--controlled

Author: nishasy

Reviewers: jeremywiebe

Required Reviewers:

Approved By: jeremywiebe

Checks: ✅ codecov/project, ✅ codecov/patch, ✅ Upload Coverage (ubuntu-latest, 20.x), ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ✅ Cypress (ubuntu-latest, 20.x), ✅ Jest Coverage (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x), ✅ gerald

Pull Request URL: #1463

.changeset/wicked-rocks-exist.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/perseus-editor": patch
+---
+
+[Interactive Graph Editor - Storybook] Fix broken default and controlled stories

packages/perseus-editor/src/widgets/interactive-graph-editor.tsx

@@ -473,7 +473,7 @@ class InteractiveGraphEditor extends React.Component<Props> {
                     </LabeledRow>
                 )}
                 {this.props.graph?.type &&
-                    this.props.apiOptions.flags?.mafs?.["start-coords-ui"] && (
+                    this.props.apiOptions?.flags?.mafs?.["start-coords-ui"] && (
                         <StartCoordsSettings
                             {...this.props.graph}
                             range={this.props.range}