hi,welcome.
you need to find the place where the widget was created(create widget,add to viewport…).i think it’s in the third person charecter blueprint.
i notice you exposed the charecter variable in the widget blueprint.
that means there should be an input pin on the “create widget node”.make sure you have plugged the charecter into it.
the error massege says that the charecte is not valid,means you didn’t assign a charecter to the variable you created in the widget blueprint. the charecter variable is currently empty. 
hope it helps