Null check operator used on a null value, null safety causing many errors

Question

First I had a problem that I fixed by changing late String myName, myProfilePic, myUserName, myEmail; to String? myName, myProfilePic, myUserName, myEmail;, that required me to add a null check operator (!) to to var chatRoomId = getChatRoomIdByUsernames(myUserName, username);, (So it changes to var chatRoomId = getChatRoomIdByUsernames(myUserName!, username); but then it gives me that error. Can anyone tell me what to do? When I don't use (!) then the error on myUserName is "The argument type 'String?' can't be assigned to the parameter type 'String'".