android.content.Context.getPackageName() on a null object reference

It seems that you're getting a wrong context that raises this NullPointerException

Try to replace the below line:

Intent mainIntent = new Intent (getContext(), MainActivity.class);

With: >> if you're within an activity

Intent mainIntent = new Intent (this, MainActivity.class);

with: >> if you're within a callback listener within the activity

Intent mainIntent = new Intent (MyActivityName.this, MainActivity.class);     

With: >> if you're within a fragment

Intent mainIntent = new Intent (requireActivity(), MainActivity.class);

Please try with getActivity() intsead of getContext()

I work the transaction of the fragment with this code

private Context context;

context.startActivity(new Intent(context, MainActivity.class)); 

I have been able to find a work around this. I realised I am getting the error because I am creating the new Intent inside the firebase OnCompleteListener as shown below. So after i removed it from inside the listener and called it outside, my program works properly.

I instead created a global boolean variable that I updated to true on successful data storage. Then I can access it somewhere else and then transtion to another if true.

 #BEFORE

    PostsRef.child(key).updateChildren(postsMap)
                                    .addOnCompleteListener(new OnCompleteListener<Void>() {
                                        @Override
                                        public void onComplete(@NonNull Task<Void> task) {
                                            if (task.isSuccessful()) {
                                                Toast.makeText(getActivity(), "New Post is updated successfully.", Toast.LENGTH_SHORT).show();
                                                Intent mainIntent = new Intent (getActivity(), MainActivity.class);
                                                mainIntent.addFlags (Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
                                                startActivity (mainIntent);
                                            } else {
                                                Toast.makeText(context, "Error occured while updating your post.", Toast.LENGTH_SHORT).show();
                                            }
                                        }
                                    });
#AFTER

 PostsRef.child(key).updateChildren(postsMap)
                            .addOnCompleteListener(new OnCompleteListener<Void>() {
                                @Override
                                public void onComplete(@NonNull Task<Void> task) {
                                    if (task.isSuccessful()) {
                                        mSuccess = true;
                                        Toast.makeText(getActivity(), "New Post is updated successfully.", Toast.LENGTH_SHORT).show();
                                    } else {
                                        Toast.makeText(context, "Error occured while updating your post.", Toast.LENGTH_SHORT).show();
                                    }
                                }
                            });

Hello guys I have also realised one of the big causes of this error is the lifecycle methods.So I was overriding them inside my fragment to update the user state on the firebase database. Because of this they caused the android.content.Context.getPackageName() on a null object reference error whenever I tried to move to the main activity managing it. Incase I used the fragment transition to move to another fragment, the MainActivity would get excecuted more than twice. After stopping to override them my application worked properly. If someone would explain why that happens it would be great.

   @Override
        public void onStart() {
            super.onStart();
    updateUserState("Online");
        }
@Override
        public void onResume() {
            super.onResume();
    updateUserState("Online");
        }

Since, you are calling from a Fragment, you should use getActivity() here instead of getContext()

Intent mainIntent = new Intent (getActivity(), MainActivity.class);
        mainIntent.addFlags (Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity (mainIntent);

This should help I hope.

Sometimes startActivity method throws an exception like:

Calling startActivity() from outside of an Activity context requires the FLAG_ACTIVITY_NEW_TASK flag. Is this really what you want?

So you should intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK) method