It seems that the "Text" column will only be filled for stored program units, not for the Anonymous PL/SQL Block of the Test Script. I think that immediately after execution it should have been available though. We'll check it out.
I have the same problem. I'm comparing the execution time for 3 runs of a procedure call. 1 run processes 1000 records, 2 10000 and 3 100000 - exactly the same call and the Total Time column is only populated for the 2nd call. My colleagues have reported the same issue on both v11 and v12 of PL/SQL developer