[Bug]: Invalid/blank mapbox URL freezes application

[battlehax]

Setting invalid URL or blank in "Application Settings">"Map Settings">"Mapbox Map Style" will freeze the application and closing/reopening the application it freezes immediately. After tracking down and deleting the app's settings folder, I was able to run it again.

Sorry I cannot submit a fix for this language. I believe a simple check and possible revert to the default URL could be a good fix for this.

Thanks for the awesome project!