Firestore does not return data in String method in Android Studio

Question

I tried to get the documents in subcollection and put it in Arraylist. Then the data in the Arraylist will return in the String method which is the load() method.The load() method must return in String elsewhere the loadPlan() method will be in error. But when I ran the app the return in the load() method is return to null. The data from the firestore is not return to it.How can I solve this. Here is the snapshot of my code and the logcat.

class ViewHolder extends RecyclerView.ViewHolder{
    TextView name;

    public ViewHolder(@NonNull View itemView) {
        super(itemView);
        name = itemView.findViewById(R.id.name);
        itemView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                final Intent intent = new Intent(getBaseContext(), DJIDashboardActivity.class);
                intent.putExtra("mapCredentialsKey", MAP_CREDENTIALS_KEY);
                startActivityIfNeeded(intent, RequestCodes.DASHBOARD);
                loadPlan();
            }
        });
    }

    public void bind(PlanModel planModel){
        name.setText(planModel.getName());


    }
}

...................

private void loadPlan() {
    try {
        Dronelink.getInstance().loadPlan(load(), false, this, (final String error) -> { Log.e(TAG, "Unable to read mission plan: " + error); });
    } catch (final Dronelink.KernelUnavailableException e) {
        Log.e(TAG, "Dronelink Kernel Unavailable");
    } catch (final Dronelink.UnregisteredException e) {
        Log.e(TAG, "Dronelink SDK Unregistered");
    }
}

 public String load() {


    TempDialog = new ProgressDialog(MainActivity.this);
    TempDialog.setMessage("Please wait ");
    TempDialog.setCancelable(false);
    TempDialog.setProgress(i);
    TempDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
    TempDialog.getWindow().setBackgroundDrawable(new ColorDrawable(Color.GRAY));

    TempDialog.show();



    mission(new FirestoreCallback() {
        @Override
        public void onCallback(List<String> list) {
            Log.d(TAG,"1" + contentList.toString());
        }
    });

    Log.d(TAG, "3 PRINT " + contentList.toString());
    return  contentList.toString();
}

public void mission(FirestoreCallback firestoreCallback) {
    db.collection("plans").document("FQQJY4Ia5xFLKExVkHtl").collection("versions").get()
            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                @Override
                public void onComplete(@NonNull Task<QuerySnapshot> task) {
                    if (task.isSuccessful()) {

                        for (QueryDocumentSnapshot doc : task.getResult()) {


                            contentList.clear();

                            String List = doc.getString("content");
                            contentList.add(List);
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                            Log.d(TAG, "FROM ARRAY "+contentList.get(0));

                        }
                        firestoreCallback.onCallback(contentList);


                    }else {
                        Log.e(TAG, "Firestore error");
                    }
                }

            });
}

private  interface FirestoreCallback{
    void onCallback(List<String> list);
}

4,5