Fix possible crash

Observed with an MFC application

Changelog

@@ -15,6 +15,7 @@
   - Introduced Config::setDropIndicatorsInhibited(), which allows you to disable support
   for drop indicators while dragging.
   - Fixed embedding the main window into a non-kddw main window (#204, #168)
+  - Fixed crash with an MFC application
 
 * v1.3.2 (unreleased)
   -

src/private/DockRegistry.cpp

@@ -202,7 +202,7 @@ bool DockRegistry::isProbablyObscured(QWindow *window, FloatingWindow *exclude)
     for (MainWindowBase *mw : m_mainWindows) {
         QWindow *mwWindow = mw->window()->windowHandle();
 
-        if (mwWindow != window && !targetIsToolWindow && mwWindow->geometry().intersects(geo)) {
+        if (mwWindow && mwWindow != window && !targetIsToolWindow && mwWindow->geometry().intersects(geo)) {
             // Two main windows that intersect. Return true. If the target is a tool window it will be above, so we don't care.
             return true;
         }