Short-circuit _debugCanPerformMutations if debugDoingLayout is false (#81246)

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

@@ -1359,11 +1359,18 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
 
   /// Whether tree mutations are currently permitted.
   ///
-  /// Only valid when asserts are enabled. In release builds, always returns
-  /// null.
+  /// This is only useful during layout. One should also not mutate the tree at
+  /// other times (e.g. during paint or while assembling the semantic tree) but
+  /// this function does not currently enforce those conventions.
+  ///
+  /// Only valid when asserts are enabled. This will throw in release builds.
   bool get _debugCanPerformMutations {
     late bool result;
     assert(() {
+      if (owner != null && !owner!.debugDoingLayout) {
+        result = true;
+        return true;
+      }
       RenderObject node = this;
       while (true) {
         if (node._doingThisLayoutWithCallback) {