Save Page (and Print to PDF) file dialogs cause ASSERTs on Linux in GTK3

Steps to reproduce:
1. Launch demobrowser
2. Right click and choose Save Page
3. In the file dialog that gets opened, click on the 3 dots to choose a file name
Expected result: File dialog opens
Actual result: The application asserts because of GTK3 errors. In the debugger, you can continue, and (after a couple of other asserts and contiues) the application will open the file dialog.