Fix assert in RenderObject.getTransformTo() (flutter#81590)

The assert was never hitting because it was first hitting a
null cast error in the `renderer.parent!` line in the for
loop.

Author: tvolkert

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

@@ -2397,8 +2397,8 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
     }
     final List<RenderObject> renderers = <RenderObject>[];
     for (RenderObject renderer = this; renderer != ancestor; renderer = renderer.parent! as RenderObject) {
-      assert(renderer != null); // Failed to find ancestor in parent chain.
       renderers.add(renderer);
+      assert(renderer.parent != null); // Failed to find ancestor in parent chain.
     }
     if (ancestorSpecified)
       renderers.add(ancestor!);

packages/flutter/test/rendering/object_test.dart

@@ -100,6 +100,15 @@ void main() {
     expect(() => data3.detach(), throwsAssertionError);
   });
 
+  test('RenderObject.getTransformTo asserts is argument is not descendant', () {
+    final PipelineOwner owner = PipelineOwner();
+    final TestRenderObject renderObject1 = TestRenderObject();
+    renderObject1.attach(owner);
+    final TestRenderObject renderObject2 = TestRenderObject();
+    renderObject2.attach(owner);
+    expect(() => renderObject1.getTransformTo(renderObject2), throwsAssertionError);
+  });
+
   test('PaintingContext.pushClipRect reuses the layer', () {
     _testPaintingContextLayerReuse<ClipRectLayer>((PaintingContextCallback painter, PaintingContext context, Offset offset, Layer? oldLayer) {
       return context.pushClipRect(true, offset, Rect.zero, painter, oldLayer: oldLayer as ClipRectLayer?);