title: users does not define no argument constructor
body: <p>i kept getting this error
  "users does not define a no-argument constructor. If you are using ProGuard, make sure these constructors are not stripped." Tried everything, no idea why it happen.</p>
  
  <pre><code>public void retrievingUserInfo(){
  
      databaseUserID.addValueEventListener(new ValueEventListener() {
          @Override
          public void onDataChange(DataSnapshot dataSnapshot) {
  
              //clearing the previous userinfo list
              Users_Info.clear();
  
              //iterating through all the nodes
              for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
                  //getting userinfo
                  users userinfo = postSnapshot.getValue(users.class);
                  //adding userinfo to the list
                  Users_Info.add(userinfo);
              }
          }
  
          @Override
          public void onCancelled(DatabaseError databaseError) {
  
          }
      });
  }
  </code></pre>
  
  <p>users.class</p>
  
  <pre><code>@Keep
  public class users {
  
      public String user_id, address, contact, name;
  
      public users(String user_id, String address, String contact,String name) 
      {}
  
  
  }
  </code></pre>