Table: Fixed potential Nullrefexception.

Exception was:
System.NullReferenceException: Object reference not set to an instance of an object.
   at XPTable.Models.Table.PaintBrokenGridColumn(PaintEventArgs e, Pen gridPen, Int32 column, Int32 x) in S:\xptable\xptable_git\Models\Table.cs:line 8200
   at XPTable.Models.Table.PaintGridColumns(PaintEventArgs e, Pen gridPen) in S:\xptable\xptable_git\Models\Table.cs:line 8164
   at XPTable.Models.Table.OnPaintGrid(PaintEventArgs e) in S:\xptable\xptable_git\Models\Table.cs:line 7950
   at XPTable.Models.Table.OnPaint(PaintEventArgs e) in S:\xptable\xptable_git\Models\Table.cs:line 7639
   at System.Windows.Forms.Control.PaintWithErrorHandling(PaintEventArgs e, Int16 layer)
   at System.Windows.Forms.Control.WmPaint(Message& m)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

Issue: SARA-2815