fix bug of NPE

[xingchueng]

05-20 16:07:32.433 4529 4529 E AndroidRuntime: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.hardware.Camera.startPreview()' on a null object reference
05-20 16:07:32.433 4529 4529 E AndroidRuntime: at org.tensorflow.lite.examples.classification.LegacyCameraConnectionFragment.onResume(LegacyCameraConnectionFragment.java:154)

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[jdduke]

Can you also change this for the other samples that have the LegacyCameraConnectionFragment class?

[MarkDaoust]

Thanks for sending a PR @xingchueng

Can you sign the CLA, and address the request from the reviewer?

[xingchueng]

I Signed it

[xingchueng]

I signed it!
Maybe I used wrong email address last time