Fixed NPE in AndroidImplementation (#4706)

https://www.reddit.com/r/cn1/comments/1r8xtxe/comment/oeizrp6/?context=1

```
Exception java.lang.NullPointerException: Attempt to invoke virtual method 'void com.codename1.impl.android.a.setPaintViewOnBuffer(boolean)' on a null object reference
  at com.codename1.impl.android.AndroidImplementation$7.run (AndroidImplementation.java:1568)
  at java.lang.Thread.run (Thread.java:1119)Exception java.lang.NullPointerException: Attempt to invoke virtual method 'void com.codename1.impl.android.a.setPaintViewOnBuffer(boolean)' on a null object reference
  at com.codename1.impl.android.AndroidImplementation$7.run (AndroidImplementation.java:1568)
  at java.lang.Thread.run (Thread.java:1119)
```

Signed-off-by: Shai Almog <67850168+shai-almog@users.noreply.github.com>

Author: shai-almog