[BUG] Some NPEs from Google Play Console

[Aitorbp]

Related Issues

App: https://github.com/owncloud/android/issues/4207

• [x] Added changelog files for the fixed issues in folder changelog/unreleased. More info here

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QA

[jesmrec]

I did some checks regarding the FileDataStorageManager and the ReceiveExternalFileActivity to check if nullability checks affect somehow. For me, we can move forward this one, waiting for the next release to check whether crash reports are not there anymore.

Moving this forward