Android Error [Attempt to Invoke Virtual Method 'Void Android.App.Actionbar' on a Null Object Reference]

Android Error [Attempt to invoke virtual method 'void android.app.ActionBar' on a null object reference]

Your code is throwing on com.example.tabwithslidingdrawer.MainActivity.onCreate(MainActivity.java:95):

        // enabling action bar app icon and behaving it as toggle button
        getActionBar().setDisplayHomeAsUpEnabled(true);
        getActionBar().setHomeButtonEnabled(true);

The problem is pretty simple- your Activity is inheriting from the new android.support.v7.app.ActionBarActivity. You should be using a call to getSupportActionBar() instead of getActionBar().

If you look above around line 65 of your code you'll see that you're already doing that:

        actionBar = getSupportActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_LIST);
        // TODO: Remove the redundant calls to getSupportActionBar()
        //       and use variable actionBar instead
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        getSupportActionBar().setHomeButtonEnabled(true);

And then lower down around line 87 it looks like you figured out the same:

        getSupportActionBar().setTitle(
                        Html.fromHtml("<font color=\"black\">" + mTitle + " - "
                                        + menutitles[0] + "</font>"));
        // getActionBar().setTitle(mTitle +menutitles[0]);

Notice how you commented out getActionBar().

Android Error : Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.hide()' on a null object reference

On your MainActivity you are trying to hide the Action Bar by using the code getActionBar().hide,

But the problem is your MainActivity doesn't have an Action Bar because your Main Activity is using the Theme called Theme.Shopit which removes the Action Bar by default because its parent is Theme.MaterialComponents.DayNight.NoActionBar.

So to fix this issue you can simply remove the code getActionBar().hide from your MainActivity.

Shwing Error : Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)

Try this

Add this in onCreate

 ActionBar actionBar = this.getSupportActionBar();
 if (actionBar != null) {
    actionBar.setDisplayHomeAsUpEnabled(true);
 }

Add this outside of the onCreate, this is used to navigate back to the parent activity

@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
   if (id == android.R.id.home) {
    NavUtils.navigateUpFromSameTask(this);
}
    return super.onOptionsItemSelected(item);
}

And also set Parent activity

  <activity
        android:name=".activity.CreateAccountActivity"
        android:parentActivityName=".activity.LoginActivity" />

How Attempt to invoke virtual method on a null object reference

You are getting NullPointerException because you are binding your recyclerView in the wrong way

in fragments you need to use an inflated view or getView() for findViewById

Use this

recyclerView = view.findViewById(R.id.userList);

instead of this

recyclerView = getActivity().findViewById(R.id.userList);

Recycler View : Attempt to invoke virtual method on a null object reference

As I understand from fragment_home.xml You have a RecyclerView inside of HomeFragment xml layout. But you are tryin to find the View inside MainActivity.
The issue in your MainActivity's onCreate method.
When you call :

RecyclerView recyclerView = findViewById(R.id.mRecyclerView);

From MainActivity It's trying to find a View with id: R.id.mRecyclerView in you activity_main.xml(xml layout that you're using for MainActivity). But as I understand it's absent there, because it's inside fragment_home.xml.
Your can read how find View::findViewById works here:

https://developer.android.com/reference/android/view/View#findViewById(int)

Finds the first descendant view with the given ID, the view itself if
the ID matches getId(), or null if the ID is invalid (< 0) or there is
no matching view in the hierarchy.

So, When it can't find the view by id, it returns null.

Then you try to set Adapter to RecyclerView's variable which actually is null.

recyclerView.setAdapter(adapter); //recyclerView is null here

That's the reason of your issue.

You have 2 options.

1. Move RecyclerView from fragment_home.xml to activity_main.xml, than it will be worked.
2. Another option more suitable in my opinion, since you've already used fragments:

Move your code related to RecyclerView handling to HomeFragment class and call findViewById inside HomeFragment.

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.io.File.exists()' on a null object reference

you get this exception because the method "getOutputMediaFile" return null and you try to use null object.

first, add null check to file, its always good approach.

second, you have to understand why this method return null and not the file you want.
if you try to use "getExternalStorageDirectory" and call mkdirs(), its depracated, you should use "getExternalFilesDir" on your context/activity.

try to do the change, if it does not help, please share with us the method "getOutputMediaFile" so we can see whats going on there and give you further help.

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