fix: null check in `ScreenRetriever._handleScreenEvent`

[MSOB7YY]

for some reason, the type in ScreenRetriever._handleScreenEvent() can be null. just added a quick null check

exception details:

type 'Null' is not a subtype of type 'String' in type cast => _TypeError
=> #0      ScreenRetriever._handleScreenEvent (package:screen_retriever/src/screen_retriever.dart:15:33)