Check to make sure lastFocus is not NULL before using it.

This prevents an app_server crash on startup for me most likely
introduced in hrev45252. (Just a few commits ago)

src/servers/app/Desktop.cpp

@@ -1914,10 +1914,12 @@ Desktop::SetFocusWindow(Window* nextFocus)
 			// If the last window having focus is a window that cannot make it
 			// to the front, we use that as the next focus
 			Window* lastFocus = fFocusList.LastWindow();
-			if (!lastFocus->SupportsFront() && _WindowCanHaveFocus(lastFocus))
+			if (lastFocus != NULL && !lastFocus->SupportsFront()
+				&& _WindowCanHaveFocus(lastFocus)) {
 				nextFocus = lastFocus;
 			}
 		}
+	}
 
 	// make sure no window is chosen that doesn't want focus or cannot have it
 	while (nextFocus != NULL && !_WindowCanHaveFocus(nextFocus)) {