having some error while using firebase firestore for fetching data

it is an exam preparation app that i am making but in the code below in the db query class i used a loadquestions method to fetch data in an array list named g_quesList. but when i used set data method in StartTest class
in totalQ i used g_quesList.size it shows 0 in questions as you can see in the screen shot where it should show the no of records. I tried stack overflow and the solution was that firebase database is asyncronous u should use call back or live data function but i dont understand how. rectify this error