Wpf: Fix crash when closing a Dialog

[cwensley]

This was a regression introduced in #2131, calling Dialog.Close() causes the following exception:

System.InvalidOperationException: 'Cannot set Owner property after Dialog is shown.'
[External Code]	
Eto.Wpf.dll!Eto.Wpf.Forms.WpfWindow<System.Windows.Window, Eto.Forms.Dialog, Eto.Forms.Window.ICallback>.SetOwner(Eto.Forms.Window owner) Line 897	C#
Eto.Wpf.dll!Eto.Wpf.Forms.WpfWindow<System.Windows.Window, Eto.Forms.Dialog, Eto.Forms.Window.ICallback>.Close() Line 412	C#
Eto.dll!Eto.Forms.Window.Close() Line 327	C#
ZooClient.dll!ZooClient.ClientManager._PerformCloudZooTask.AnonymousMethod__5() Line 1952	C#
[External Code]	
Eto.Wpf.dll!Eto.Wpf.Forms.DialogHandler.ShowModal() Line 62	C#
Eto.dll!Eto.Forms.Dialog.ShowModal() Line 225	C#
Eto.dll!Eto.Forms.Dialog.ShowModal(Eto.Forms.Control owner) Line 207	C#
Eto.dll!Eto.Forms.Dialog<bool>.ShowModal(Eto.Forms.Control owner) Line 87	C#