After clicking the Coqtool icon, a splash screen appears and then nothing.
Indeed, I encountered the same error as you did. The application starts, the splash screen is displayed then nothing. However, there is a version that works properly. I've tested it the error is no more.
Version with error: 1.3dev.
Version stable: 1.2.2 [Download: https://github.com/cogtool/cogtool/releases/tag/1.2.2 ]
Here is a proof that it works: