introduce a new StartShutdown() function, which starts a thread with Shutdown() if no GUI is used and calls uiInterface.QueueShutdown() if a GUI is used / all direct uiInterface.QueueShutdown() calls are replaced with Shutdown() - this ensures a clean GUI shutdown, even when catching a SIGTERM and allows the BitcoinGUI destructor to get called (which fixes a tray-icon issue and keeps the tray-icon until Bitcoin-Qt exits)

src/main.cpp

@@ -1875,7 +1875,7 @@ bool CheckDiskSpace(uint64 nAdditionalBytes)
         strMiscWarning = strMessage;
         printf("*** %s\n", strMessage.c_str());
         uiInterface.ThreadSafeMessageBox(strMessage, "Bitcoin", CClientUIInterface::OK | CClientUIInterface::ICON_EXCLAMATION | CClientUIInterface::MODAL);
-        uiInterface.QueueShutdown();
+        StartShutdown();
         return false;
     }
     return true;