Return "invalid" stores from scoping (#2601)

* Return "invalid" stores from scoping

Scoping can sometimes be invalid, _e.g._ SwiftUI may scope through an
index of a collection that no longer exists while diffing in a
`ForEach`, which can cause a crash when the child state is eagerly
evaluated. This commit introduces the idea of an invalid store that will
only crash if the store's state is interacted with, avoiding crashes
that could happen deeper in SwiftUI.

* wip

Sources/ComposableArchitecture/Store.swift

@@ -139,7 +139,7 @@ public final class Store<State, Action> {
   private var isSending = false
   var parentCancellable: AnyCancellable?
   private let reducer: any Reducer<State, Action>
-  @_spi(Internals) public var stateSubject: CurrentValueSubject<State, Never>
+  @_spi(Internals) public var stateSubject: CurrentValueSubject<State, Never>!
   #if DEBUG
     private let mainThreadChecksEnabled: Bool
   #endif
@@ -175,6 +175,14 @@ public final class Store<State, Action> {
     }
   }
 
+  fileprivate init() {
+    self._isInvalidated = { true }
+    self.reducer = EmptyReducer()
+    #if DEBUG
+      self.mainThreadChecksEnabled = true
+    #endif
+  }
+
   deinit {
     self.invalidate()
     Logger.shared.log("\(storeTypeName(of: self)).deinit")
@@ -994,12 +1002,15 @@ private final class ScopedStoreReducer<RootState, RootAction, State, Action>: Re
 
   @inlinable
   func reduce(into state: inout State, action: Action) -> Effect<Action> {
-    if self.isInvalid() {
+    let isInvalid = self.isInvalid()
+    if isInvalid {
       self.onInvalidate()
     }
     self.isSending = true
     defer {
-      state = self.toState(self.rootStore.stateSubject.value)
+      if !isInvalid || state is _OptionalProtocol {
+        state = self.toState(self.rootStore.stateSubject.value)
+      }
       self.isSending = false
     }
     if let action = self.fromAction(action),
@@ -1032,6 +1043,10 @@ extension ScopedStoreReducer: AnyScopedStoreReducer {
     isInvalid: ((S) -> Bool)?,
     removeDuplicates isDuplicate: ((ChildState, ChildState) -> Bool)?
   ) -> Store<ChildState, ChildAction> {
+    guard isInvalid.map({ $0(store.stateSubject.value) == false }) ?? true
+    else {
+      return Store()
+    }
     let id = id?(store.stateSubject.value)
     if let id = id,
       let childStore = store.children[id] as? Store<ChildState, ChildAction>
@@ -1053,7 +1068,7 @@ extension ScopedStoreReducer: AnyScopedStoreReducer {
     }
     let reducer = ScopedStoreReducer<RootState, RootAction, ChildState, ChildAction>(
       rootStore: self.rootStore,
-      state: { [stateSubject = store.stateSubject] _ in toChildState(stateSubject.value) },
+      state: { [stateSubject = store.stateSubject!] _ in toChildState(stateSubject.value) },
       action: { fromChildAction($0).flatMap(fromAction) },
       isInvalid: isInvalid,
       onInvalidate: { [weak store] in