qtwidgets|MDI: Fix showing resize handles

if there was no frame being resized it would be nullptr, and hence
enter that block.

QtQuick not affected since it uses mouse areas to change the cursor
shape

src/private/WidgetResizeHandler.cpp

@@ -133,9 +133,13 @@ bool WidgetResizeHandler::eventFilter(QObject *o, QEvent *e)
         if (mTarget->isMaximized())
             break;
 
-        if (isMDI() && DockRegistry::self()->frameInMDIResize() != mTarget) {
-            // Some other frame is being resized.
-            return false;
+        if (isMDI()) {
+            const Frame *frameBeingResized = DockRegistry::self()->frameInMDIResize();
+            const bool otherFrameBeingResized = frameBeingResized && frameBeingResized != mTarget;
+            if (otherFrameBeingResized) {
+                // only one at a time!
+                return false;
+            }
         }
 
         auto mouseEvent = static_cast<QMouseEvent *>(e);