WPF/OffScreen - Avoid ObjectDisposedException when browser is already disposed

[campersau]

Summary:

• When opening popups which close themself (e.g. with window.close()) it can happen that the IBrowser is already disposed but not the ChromiumWebBrowser, which can lead to ObjectDisposedException in dispose.

Changes:

• Added a check for IBrowser.IsDisposed before accessing it.

How Has This Been Tested?
Manually. Unfortunately I wasn't able to create a test for it yet.

Screenshots (if appropriate):

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Updated documentation

Checklist:

• [X] Tested the code(if applicable)
• [ ] Commented my code
• [ ] Changed the documentation(if applicable)
• [ ] New files have a license disclaimer
• [ ] The formatting is consistent with the project (project supports .editorconfig)

[AppVeyorBot]

:white_check_mark: Build CefSharp 128.4.90-CI5063 completed (commit https://github.com/cefsharp/CefSharp/commit/3c0908f894 by @campersau)

[amaitland]

Thanks 👍 Check seems sensible, hopefully that's enough, otherwise it'll have to be a catch (ObjectDisposedException