It seems to be a communication issue between my app and the Firebase database.

MIT App Inventor Help
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I have built an app that reads data from a Firebase Realtime Database. While it was working perfectly fine, it suddenly crashes on every screen that attempts to read data, displaying the following message:

'Error from Companion: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference'"

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Use the Firebase RealtimeDatabase REST api with the AppInventor Web component.

I have an old project created in 2019 that still works OK with the native firebase component, so if you also have an old project...

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Or use FirebaseManager extension, [FREE] FirebaseManager Extension (Updated)