[Bug] NRE thrown on purge all cache files

[GreenKeldeo]

Background

• Operating System: Windows 11 Home 64-bit (10.0, Build 22621) (22621.ni_release.220506-1250)
• CKAN Version: 1.31.1
• KSP Version: 1.12.5 (Although I don't think it matters here)

Have you made any manual changes to your GameData folder (i.e., not via CKAN)? Yes, but not to the CKAN cache - nor do I have the CKAN cache open elsewhere.

Problem

Describe the bug
With a 12.8gb CKAN cache, purge all throws an NRE.

Steps to reproduce

• Open CKAN
• Go to CKAN settings
• Press purge
• Select purge all

Expected behavior
Cache files are purged

Screenshots (if applicable)

CKAN error code (if applicable): N/A; Exception text can be found below:

See the end of this message for details on invoking 
just-in-time (JIT) debugging instead of this dialog box.

************** Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
   at CKAN.GUI.Contents._UpdateModContentsTree(CkanModule module, Boolean force)
   at CKAN.GUI.SettingsDialog.PurgeAllMenuItem_Click(Object sender, EventArgs e)
   at System.Windows.Forms.ToolStripItem.RaiseEvent(Object key, EventArgs e)
   at System.Windows.Forms.ToolStripMenuItem.OnClick(EventArgs e)
   at System.Windows.Forms.ToolStripItem.HandleClick(EventArgs e)
   at System.Windows.Forms.ToolStripItem.HandleMouseUp(MouseEventArgs e)
   at System.Windows.Forms.ToolStrip.OnMouseUp(MouseEventArgs mea)
   at System.Windows.Forms.ToolStripDropDown.OnMouseUp(MouseEventArgs mea)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ToolStrip.WndProc(Message& m)
   at System.Windows.Forms.ToolStripDropDown.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

[HebaruSan]

This should be an easy one. PurgeAllMenuItem_Click passes null:

CKAN/GUI/Dialogs/SettingsDialog.cs

Lines 266 to 267 in efb7fc8

	// finally, clear the preview contents list
	Main.Instance.UpdateModContentsTree(null, true);

and _UpdateModContentsTree tries to access a property on it:

CKAN/GUI/Controls/ModInfoTabs/Contents.cs

Lines 58 to 71 in efb7fc8

	private void _UpdateModContentsTree(CkanModule module, bool force = false)
	{
	ContentsPreviewTree.BackColor = SystemColors.Window;
	ContentsPreviewTree.LineColor = SystemColors.WindowText;
	if (Equals(module, currentModContentsModule) && !force)
	{
	return;
	}
	else
	{
	currentModContentsModule = module;
	}
	if (module.IsMetapackage)