Fixed null ref in FacebookActivity
Fixed null ref in FacebookActivity

Summary: See title

Test Plan:
- unit tests

Revert Plan:
- safe

Tags:

Reviewers: clang, mingfli

Reviewed By: clang

CC: msdkexp@

Differential Revision: https://phabricator.fb.com/D633256

Task ID: 1863836

minhduc3404/facebookSdk,RabbleApp/facebook-android-sdk,TechSmith/facebook-android-sdk,MKrishtop/facebook-android-sdk,scalio/facebook-android-sdk,RabbleApp/facebook-android-sdk,MKrishtop/facebook-android-sdk,CristianOliveiraDaRosa/facebook-android-sdk,PioneerLab/facebook-android-sdk,scalio/facebook-android-sdk,GrioSF/facebook-android-sdk,CristianOliveiraDaRosa/facebook-android-sdk,bitcellar-labs/facebook-android-sdk,TimeIncOSS/facebook-android-sdk,taisukeoe/facebook-android-sdk,lizhi5753186/facebook-android-sdk,couchbaselabs/facebook-android-sdk,ribot/facebook-android-sdk,Flutterbee/facebook-android-sdk,Flutterbee/facebook-android-sdk,mindjolt/facebook-android-sdk,PioneerLab/facebook-android-sdk,Limbika/facebook-android-sdk,satoshi-okawa/facebook-android-sdk-test1,taisukeoe/facebook-android-sdk,MarkHappy/Facebook_SDK,mindjolt/facebook-android-sdk,TimeIncOSS/facebook-android-sdk,minhduc3404/facebookSdk,TechSmith/facebook-android-sdk,quizlet/facebook-android-sdk,Limbika/facebook-android-sdk,satoshi-okawa/facebook-android-sdk-test1,ribot/facebook-android-sdk,quizlet/facebook-android-sdk,MarkHappy/Facebook_SDK,lizhi5753186/facebook-android-sdk,couchbaselabs/facebook-android-sdk,bitcellar-labs/facebook-android-sdk,GrioSF/facebook-android-sdk