Don't try to update filters if mod list missing

[HebaruSan]

Problem

Since #2617, we occasionally get exceptions at startup:

System.NullReferenceException: Object reference not set to an instance of an object.
   at CKAN.Main._UpdateFilters()
   at CKAN.Util.Invoke[T](T obj, Action action)
   at CKAN.Main.UpdateFilters(Main control)
   at CKAN.Main.<.ctor>b__37_0(MainModList source)
   at CKAN.MainModList.set_ModFilter(GUIModFilter value)
   at CKAN.Main.Filter(GUIModFilter filter)
   at CKAN.Main.CurrentInstanceUpdated()
   at CKAN.Main.OnLoad(EventArgs e)
   at System.Windows.Forms.Form.OnCreateControl()
   at System.Windows.Forms.Control.CreateControl(Boolean fIgnoreVisible)
   at System.Windows.Forms.Control.CreateControl()
   at System.Windows.Forms.Control.WmShowWindow(Message& m)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.Form.WmShowWindow(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

Cause

Now that UpdateModsList runs in a background thread, things that formerly happened afterwards can now run alongside it, including several places that populate the mod filters in the GUI. This calls UpdateFilters, which crashes here if UpdateModsList hasn't set mainModList.full_list_of_mod_rows yet:

CKAN/GUI/MainModList.cs

Line 109 in 1bbd2fd

var rows = new DataGridViewRow[mainModList.full_list_of_mod_rows.Count];

Changes

Now UpdateFilters treats the existence of mainModList.full_list_of_mod_rows as a prerequisite and quits if it's null.

Fixes #2650.