[mob][photos] Assert that SelectionState inherited widget is in context or not

mobile/lib/ui/viewer/gallery/state/selection_state.dart

@@ -18,10 +18,16 @@ class SelectionState extends InheritedWidget {
     required Widget child,
   }) : super(key: key, child: child);
 
-  static SelectionState? of(BuildContext context) {
+  static SelectionState? maybeOf(BuildContext context) {
     return context.dependOnInheritedWidgetOfExactType<SelectionState>();
   }
 
+  static SelectionState of(BuildContext context) {
+    final SelectionState? result = maybeOf(context);
+    assert(result != null, 'No SelectionState found in context');
+    return result!;
+  }
+
   @override
   bool updateShouldNotify(covariant InheritedWidget oldWidget) {
     return false;