Issue title: Q_ASSERT in InspectorServerQt
 Issue body: Steps to reproduce:

1. Open remote WebInspector page
2. And close it immediately
3. Q_ASSERT on https://github.com/annulen/webkit/blob/qtwebkit-stable/Source/WebKit/qt/WebCoreSupport/InspectorServerQt.cpp#L312 will fail.
 Comments: 
 Comment 0: Does it reproduce with legacy QtWebKit?
 Comment 1: Yes.
 Comment 2: If you identified source of issue, maybe you can provide fix?
{'source': 'GithubIssues', 'repo_name_with_owner': 'annulen/webkit', 'issue_id': 196217882, 'repo_stars': 291.0}