gui: Delete progress dialog instead of hidding it

src/qt/walletcontroller.h

@@ -96,6 +96,7 @@ protected:
     QObject* worker() const { return m_wallet_controller->m_activity_worker; }
 
     void showProgressDialog(const QString& label_text);
+    void destroyProgressDialog();
 
     WalletController* const m_wallet_controller;
     QWidget* const m_parent_widget;