Error Firebase ''the previous value was empty''

I wanted to ask for your help with this problem that has been presenting me several times while using firebase in app inventor

many times it works without problem but there are days or weeks where the same code generates the error "Previous value was empty" when try use firebase

Have a look at these community search results:

previous value was empty

it will be one of the solutions found there.


maybe the reason is that you are trying to Append a value to a Tag which doesn't exist.