Fix LegacySavedStateHandleController null pointer exception when th…

…e store is cleared during recreation

* `viewModelStore.keys()` returns a copy of the current keys, but `ViewModelStore.clear()` can be called before the for-each loop finishes. That would result in a null pointer exception because the returned `viewModel` is null, but we force-cast it to a non-null value with the double-bang (!!) operator.
* There is no risk in skipping null view models, as a store that has been cleared should not be attached to a `SavedStateHandle`, and any state would be cleared.

Test: N/A
Bug: 367431234
Change-Id: I6f16881b2f8dead781aa52a58c2652c8e40a35d9

lifecycle/lifecycle-viewmodel-savedstate/src/androidMain/kotlin/androidx/lifecycle/LegacySavedStateHandleController.android.kt

@@ -76,14 +76,14 @@ internal object LegacySavedStateHandleController {
     internal class OnRecreation : SavedStateRegistry.AutoRecreated {
         override fun onRecreated(owner: SavedStateRegistryOwner) {
             check(owner is ViewModelStoreOwner) {
-                ("Internal error: OnRecreation should be registered only on components " +
-                    "that implement ViewModelStoreOwner")
+                "Internal error: OnRecreation should be registered only on components " +
+                    "that implement ViewModelStoreOwner. Received owner: $owner"
             }
-            val viewModelStore = (owner as ViewModelStoreOwner).viewModelStore
+            val viewModelStore = owner.viewModelStore
             val savedStateRegistry = owner.savedStateRegistry
             for (key in viewModelStore.keys()) {
-                val viewModel = viewModelStore[key]
-                attachHandleIfNeeded(viewModel!!, savedStateRegistry, owner.lifecycle)
+                val viewModel = viewModelStore[key] ?: continue
+                attachHandleIfNeeded(viewModel, savedStateRegistry, owner.lifecycle)
             }
             if (viewModelStore.keys().isNotEmpty()) {
                 savedStateRegistry.runOnNextRecreation(OnRecreation::class.java)