NavigatorWindow crashes application when double clicking on file option

[madareklaw]

Seen in v4.72.1

Steps to reproduce using MVVMTestApp from this repro.

• Start MVVMTestApp and add a couple of new documents via File menu -> new
• Press control tab to open the NavigatorWindow
• Whilst still holding down control double click on one of the unselected 'noName' documents. This will then crash with an InvalidOperationException

[WilliamWindesheim]

I believe this problem was introduced in 4.70.0 by this PR: #324
This also broke switching to anchorables using keyboard navigation through the NavigatorWindow