Reset m_delayFocusWindow in Workspace::cancelDelayFocus()

CCBUG: 459860


(cherry picked from commit d2899928)

src/workspace.cpp

@@ -1581,6 +1581,7 @@ void Workspace::cancelDelayFocus()
 {
     delete delayFocusTimer;
     delayFocusTimer = nullptr;
+    m_delayFocusWindow = nullptr;
 }
 
 bool Workspace::checkStartupNotification(xcb_window_t w, KStartupInfoId &id, KStartupInfoData &data)