java.lang.String android.net.Uri.toString() on a null object reference, LogCat shows this error at line 202

I am trying to get the download Url based on the condition that the image path for the storage reference is null or not.

I tried to put the whole code inside the if else statement instead of just assigning the value to download_uri inside the condition which resolves the error. I am not able to understand why this approach works and another one doesn't.

    mSaveButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            final String name=mProfileName.getText().toString();
            if(!TextUtils.isEmpty(name) && mainImageUri!=null){
                mProgressBar.setVisibility(View.VISIBLE);
                if(isChanged){
                //The task of storing the data goes through uploading the image as well.

            user_id = mUser.getUid();

            final StorageReference image_path= mStorageReference.child("profile_photo").child(user_id+".jpg");
            image_path.putFile(mainImageUri).addOnCompleteListener(new OnCompleteListener<UploadTask.TaskSnapshot>() {
                @Override
                public void onComplete(@NonNull Task<UploadTask.TaskSnapshot> task) {
                         if(task.isSuccessful()){
                            storeFireStore(image_path,name);
                         }else{
                             String error = task.getException().getMessage();
                             Toast.makeText(SetUpActivity.this,"Image Error: "+error,Toast.LENGTH_SHORT).show();

                         }
                    mProgressBar.setVisibility(View.INVISIBLE);

                }
            });

        }else{
                    //The task of storing the data does not go through uploading the image.
                    storeFireStore(null,name);
                    mProgressBar.setVisibility(View.INVISIBLE);
                }
            }

        }
    });

}

private void storeFireStore(StorageReference image_path, final String name) {
    final Uri[] download_uri = new Uri[1];
    if(image_path!=null){
        image_path.getDownloadUrl().addOnSuccessListener(new OnSuccessListener<Uri>() {
            @Override
            public void onSuccess(Uri uri) {
                download_uri[0] =uri;
                Log.d(TAG,"Download Url obtained");
            }
        });
        } else{
        download_uri[0] =mainImageUri;
        Log.d(TAG,"Download Url obtained in else");
    }

Map<String,String> userMap=new HashMap<>();
userMap.put("image", download_uri[0].toString());//line 202 this is 
where the error occurs.userMap.put("name",name);

I expect to get the downLoad_uri but it provides a null value

Source: View source