Don't crash in wxWebView under OS X if custom URI is invalid.

Don't assume that wxWebViewHandler::GetFile() always returns a valid pointer,
it may fail and return NULL, e.g. for a non-existent VFS location.

Closes #15254.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74239 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775