Auto Closing NTWindows

[Human#102]

Hello fellow humans,
I've developed a simple custom message window using NTWindow.
What I would like to do is to close it while one is still open, how can I achieve that?
The code calling the window looks like this:

if (ShowPullbackPopUp)
                Globals.RandomDispatcher.InvokeAsync(new Action(() =>
                {
                    PopUp = new PopUpWindow()
                    {
                        Caption = "SuperTrenderMA Pullback Alert",
                        TargetIndicator = this,
                        message = message
                    };
                    
                    PopUp.Show(); // open the window
                    PopUp.Activate(); // bring to the top
                }));​

I've tried different checks to see if the window PopUp is not null, still open etc. and if so close it with PopUp.Close(), but to no avail.

Edit:
I've managed to figure out some solution:

if (ShowPullbackPopUp)
                Globals.RandomDispatcher.InvokeAsync(new Action(() =>
                {
                    if (PopUp != null)
                        PopUp.Close();
                    
                    PopUp = new PopUpWindow()
                    {
                        Caption = "SuperTrenderMA Pullback Alert",
                        TargetIndicator = this,
                        message = message
                    };
                    
                    PopUp.Show(); // open the window
                    PopUp.Activate(); // bring to the top
                }));​

I've only tried null checks or alike outside the InvokeAsync method, but inside it strangely enough works, but also only during "realtime" data, not when initially computing the historical data. This isn't an issue for me since I filter out historical data anyways, but still odd.