NullReferenceException from BuildWindow when using AvalonDock

* .NET Core Version: 4.7.2
* Windows version: 1809 (OS Build 17763.557)
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Not tested

**Problem description:** NullReferenceException thrown at System.Windows.Interop.HwndHost.BuildWindow() (using AvalonDock)

**Actual behavior:**
See attached file for callstack: https://github.com/dotnet/wpf/files/3329154/BuildWindowNullReferenceException.txt

**Expected behavior:** No crash, no NullReferenceException thrown.

**Minimal repro:** No stable repro at this time. Most probable repro is saving files opened in the application. The files are represented by tabs docked in the app.

For dump see:
https://mega.nz/#!Xu4lWajL!XkARTj0SAS8ozk79gp-pMq-BYe8E6zGQd4XrlyQ4ohw