fixed a runtime issue caused by a missing form property.

src/gui/UFRMMainForm.pas

@@ -70,6 +70,7 @@ type
     tbtnConnectivity: TToolButton;
     tbtnSync: TToolButton;
     tbtnWalletLock: TToolButton;
+    procedure FormClose(Sender: TObject; var CloseAction: TCloseAction);
     procedure FormCloseQuery(Sender: TObject; var CanClose: boolean);
     procedure FormCreate(Sender: TObject);
     procedure FormDestroy(Sender: TObject);
@@ -168,6 +169,11 @@ begin
   end;
 end;
 
+procedure TFRMMainForm.FormClose(Sender: TObject; var CloseAction: TCloseAction);
+begin
+  // do not delete even if empty
+end;
+
 procedure TFRMMainForm.CM_Terminate(var Msg: TMessage);
 begin
   TUserInterface.ExitApplication;