Fix NullPointerException in handler callback

[Danopie]

On our production app, we encountered this issue:

Fatal Exception: java.lang.NullPointerException: Attempt to invoke virtual method 'void io.flutter.plugin.common.MethodChannel.invokeMethod(java.lang.String, java.lang.Object)' on a null object reference
       at com.flutter.moum.screenshot_callback.ScreenshotCallbackPlugin$1$1.run(ScreenshotCallbackPlugin.java:64)
       at android.os.Handler.handleCallback(Handler.java:942)
       at android.os.Handler.dispatchMessage(Handler.java:99)
       at android.os.Looper.loopOnce(Looper.java:201)
       at android.os.Looper.loop(Looper.java:288)
       at android.app.ActivityThread.main(ActivityThread.java:8061)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:703)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:923)

This PR fixes the above issue by making sure channel is non null in the handler callback.

[Danopie]

@beygee would you mind taking a look at this?