While I'm debugging a peice of code, I found that the debug buttons are inactive, so I cannot step into the code, after restarting the debug process again the buttons became active, but after stepping into code the following message received:
An internal error occurred during: "Execute Debug Command".
java.lang.NullPointerException
This error happens when have variables from other functions in the Expressions tab, so to fix it, simply remove any expression that has a varibale from any other function, and restart the debug session again.