Could not close hidden panel (#1554)

The following error prevents the panel to be closed.

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System.NullReferenceException: Object reference not set to an instance of an object.

at DockablePanel.Controls.DockablePanel.HiddenPanelStack.OnMouseDown(MouseEventArgs e)

at System.Windows.Forms.Control.WmMouseDown(Message& m, MouseButtons button, Int32 clicks)

at System.Windows.Forms.Control.WndProc(Message& m)

at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

OF=SoftwareIdeasModeler.MainForm

NoCmds

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Reproduce steps:

1. Auto-hide Project or Documentation panel.

2. Open up one of the panel, click the square "Click to pin this panel"

3. Double click the X to close the panel. Panel will be hidden back to the side instead of closing

4. Now click to open any hidden panel. ERROR, no hidden panels can be open. Have to restart the application to be able to open the hidden panels.