Null Pointer Exception on getGraphics()

hop of those help? Your NPE is probably not coming from getGraphics but from trying to dereference a null image variable. If your component is not displayable, createImage returns null.

help you fix your problem You have unfortunately configured a lot of your code in a very unorthodox manor. For a start Slick2d will automatically thread the execution of your program and therefore you do not need to program a thread yourself.
You also appear to be calling methods which do not exist, for example init(container) is not defined in game. Additionally, you are storing container as a data value which is not required thus why you are getting a NPE for the graphics context.

Hope this helps In the loop body, current = current.myNext; has no test for myNext being null so on either the first (or a subsequent) loop iteration current must be null. You could change

while (current.myData != null) {

while (current != null && current.myData != null) {

Hope this helps Code , Here is how you can rewrite it:

if(dataSnapshot != null && dataSnapshot.hasChild("Email") && dataSnapshot.child("Email").getValue() != null ){
    //Your code...
}