Fix crash in debugDumpSemanticsTree when transform == null (#11807)

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

@@ -736,8 +736,8 @@ class SemanticsNode extends AbstractNode {
   }
 
   static int _geometryComparator(SemanticsNode a, SemanticsNode b) {
-    final Rect rectA = MatrixUtils.transformRect(a.transform, a.rect);
-    final Rect rectB = MatrixUtils.transformRect(b.transform, b.rect);
+    final Rect rectA = a.transform == null ? a.rect : MatrixUtils.transformRect(a.transform, a.rect);
+    final Rect rectB = b.transform == null ? b.rect : MatrixUtils.transformRect(b.transform, b.rect);
     final int top = rectA.top.compareTo(rectB.top);
     return top == 0 ? rectA.left.compareTo(rectB.left) : top;
   }

packages/flutter/test/rendering/semantics_test.dart

@@ -92,6 +92,24 @@ void main() {
       expect(root.debugSemantics.getSemanticsData().actions, expectedActions);
     });
   });
+
+  test('toStringDeep() does not throw with transform == null', () {
+    final SemanticsNode child2 = new SemanticsNode();
+    final SemanticsNode child1 = new SemanticsNode();
+    final SemanticsNode root = new SemanticsNode();
+    root.addChildren(<SemanticsNode>[child1, child2]);
+    root.finalizeChildren();
+
+    expect(root.transform, isNull);
+    expect(child1.transform, isNull);
+    expect(child2.transform, isNull);
+
+    expect(root.toStringDeep(DebugSemanticsDumpOrder.traversal),
+        'SemanticsNode(8 (STALE; owner=null); Rect.fromLTRB(0.0, 0.0, 0.0, 0.0))\n'
+        ' ├SemanticsNode(7; Rect.fromLTRB(0.0, 0.0, 0.0, 0.0))\n'
+        ' └SemanticsNode(6; Rect.fromLTRB(0.0, 0.0, 0.0, 0.0))\n'
+    );
+  });
 }
 
 class TestRender extends RenderProxyBox {