Previous value was empty

Hello, I have problems with firebase, it reports a "previous value was empty." using the function "firstremove" when the value I am requesting in my firebase exists and is a list and has elements, just after the error I refresh the screen and it works for me but when compiling it keeps generating the same error, when I catch the error and I request again closes the screen without leaving any trace.

The strangest thing is that days ago the same code worked for me perfectly and compiled the apk without problem but then it stopped working without any change.

I backed up the charge to the platform but ohh surprise it doesn't work anymore it presents the same error.

if you could help me i would appreciate it