There is already a post on the forum, about that, but I feel that I need to communicate a bit more.
This is a bug in Beta2, that any package using the toolbox will get when running in Debug, and you know that graphical modeling DSLs use the toolbox.
The problem only shows up when you run your DSL under the debugger, so if you had run the DSL without debugging (Ctrl+F5) you would not have seen the problem. So the first thing is, when you don’t need to debug, please, run without debugging.
Now, you have already pressed F5, how to see the toolbox? In fact you need to remove the toolbox cache, that is, after closing the experimental instance, you can:
Next time you will debug (F5), the toolbox will be there (at least until you change the model, which will rebuild the toolbox cache)