qt: Add null check in setClientModel(0)

Don't clear tray icon menu if it was never created.
Necessary precaution after #4649.

src/qt/bitcoingui.cpp

@@ -455,8 +455,11 @@ void BitcoinGUI::setClientModel(ClientModel *clientModel)
     } else {
         // Disable possibility to show main window via action
         toggleHideAction->setEnabled(false);
-        // Disable context menu on tray icon
+        if(trayIconMenu)
-        trayIconMenu->clear();
+        {
+            // Disable context menu on tray icon
+            trayIconMenu->clear();
+        }
     }
 }