Do not assert callbacks contains key if disposed (#104292)

packages/flutter/lib/src/rendering/layer.dart

@@ -214,7 +214,7 @@ abstract class Layer extends AbstractNode with DiagnosticableTreeMixin {
       }());
     };
     return () {
-      assert(_callbacks.containsKey(callbackId));
+      assert(debugDisposed || _callbacks.containsKey(callbackId));
       _callbacks.remove(callbackId);
       _updateSubtreeCompositionObserverCount(-1);
     };

packages/flutter/test/rendering/layers_test.dart

@@ -965,6 +965,21 @@ void main() {
     expect(root.subtreeHasCompositionCallbacks, false);
     expect(a1.subtreeHasCompositionCallbacks, false);
   });
+
+  test('Double removing a observe callback throws', () {
+    final ContainerLayer root = ContainerLayer();
+    final VoidCallback callback = root.addCompositionCallback((_) { });
+    callback();
+
+    expect(() => callback(), throwsAssertionError);
+  });
+
+  test('Removing an observe callback on a disposed layer does not throw', () {
+    final ContainerLayer root = ContainerLayer();
+    final VoidCallback callback = root.addCompositionCallback((_) { });
+    root.dispose();
+    expect(() => callback(), returnsNormally);
+  });
 }
 
 class FakeEngineLayer extends Fake implements EngineLayer {