Change BeamerProvider child from Widget to Widget? for use in MultiProvider

[felixdollack]

Is your feature request related to a problem? Please describe.
I am using beamer in combination with the bloc package. I am using MultiProvider and was trying to put BeamerProvider in a list of providers but got an error because the child is of type Widget (mandatory) instead of Widget? (optional).

Describe the solution you'd like
Simple fix is to change the child of BeamerProvider to Widget? (I guess) to make this work.

Describe alternatives you've considered
As a current workaround I am wrapping the MultiProvider with the BeamerProvider.

Additional context
Personal opinion: I guess it does work as is, it just doesn't feel "clean".

I would be willing to contribute, just would want quick feedback as to not waste the effort ;)

Thank you @slovnicki for building beamer and for your consideration.