Fixes NullPointerExceptions due to callbacks in MainActivity

[sonusourav]

Fixes #891 #895

Issues #891 and #895 have a common root to callbacks which gives NullPointerExceptions.
Changes:

1. Added a function named CheckNull.
2. CheckNull checks for NullPointerException for views in MainActivity.

Screenshots

[soloturn]

is this overlapping with #1038 ?

[soloturn]

checks for null are there. if i view such code i understand @abdulwd suggesting to migrate to kotlin ;)

[soloturn]

@sonusourav @abdulwd @prajurock is there a design problem as we need such null pointer checks all over the place? i fell over a comment saying:
"Once setContentView() has been called, you will never get a null View provided you are looking in the correct layout and the View exists in that layout."

https://stackoverflow.com/questions/16437050/checking-if-a-view-null-before-setting-onclicklistener

[harshika-kashyap]

Changes look good except one point for discussion.

[harshika-kashyap]

In my opinion, encapsulating such small functionality in a separate function is overkill. You are anyways going to write if(checkNull(view_name)){}. Why not just if(view_name != null). Every function call has an overhead and hence I am not in favor of it. If readability is your main intention, then we might leave it like you have done but I find explicit check equally readable.

[harshika-kashyap]

@soloturn Hi, So I went through the codebase a bit more after going through the Stackoverflow link. I must say that I might be wrong since I started yesterday only and hence am relatively new to this project.

I think the problem is indeed a design issue.

1. We open the SettingsActivity to change settings.
2. We change a few things which requires us to restart the MainActivity. The onActivityResult(Line 1417 - MainActivity.java) method handles the callback using this snippet

case REQUEST_PREFERENCES:
        if (resultCode == RESULT_RESTART) {
          startActivity(new Intent(MainActivity.this, MainActivity.class));
          finish();
        }
        ... more code.

3. The Activity is recreated. While the Activity is getting recreated(not yet finished), the KiwixWebChromeClient uses the callback to update the view using methods such as webViewProgressChanged. Now because the views are still being recreated, we get a NPE and hence crash.

[soloturn]

@harshika-arya thanks for having a look! how would you recommend to implement this?