[Notifications] Check for corona to avoid crash

My previous attempt only fixed the case when notifications were directly in a panel.
Now it also no longer crashes when removing a panel that has a System Tray with the
notifications applet in it.

BUG: 378508
FIXED-IN: 5.8.8

Differential Revision: https://phabricator.kde.org/D6653

applets/notifications/lib/notificationsapplet.cpp

@@ -63,7 +63,7 @@ void NotificationsApplet::onScreenChanges()
 {
     // when removing the panel the applet is in, the containment is being destroyed but its corona is still
     // there, rightfully emitting availableScreenRectChanged and then we blow up if we try to access it.
-    if (!containment()) {
+    if (!containment() || !containment()->corona()) {
         return;
     }