qt: Prevent thread/memory leak on exiting RPCConsole

Make ownership of the QThread object clear, so that the RPCConsole
can wait for the executor thread to quit before shutdown is called. This
increases overall thread safety, and prevents some objects from leaking
on exit.

src/qt/bitcoingui.cpp

@@ -511,6 +511,13 @@ void BitcoinGUI::setClientModel(ClientModel *_clientModel)
             // Disable context menu on tray icon
             trayIconMenu->clear();
         }
+        // Propagate cleared model to child objects
+        rpcConsole->setClientModel(nullptr);
+#ifdef ENABLE_WALLET
+        walletFrame->setClientModel(nullptr);
+#endif // ENABLE_WALLET
+        unitDisplayControl->setOptionsModel(nullptr);
+        connectionsControl->setClientModel(nullptr);
     }
 }
 
@@ -1242,7 +1249,5 @@ void NetworkToggleStatusBarControl::mousePressEvent(QMouseEvent *event)
 /** Lets the control know about the Client Model */
 void NetworkToggleStatusBarControl::setClientModel(ClientModel *_clientModel)
 {
-    if (_clientModel) {
-        this->clientModel = _clientModel;
-    }
+    this->clientModel = _clientModel;
 }