Yes, it is possible to apply for the program while you are still in your bachelor. Therefore, you have to verify that you need less than or equal to 30 ECTS to finish your bachelor. Additionally, you have to state the missing courses and the expected bachelor grade. If you are accepted to the program, you have to prove that you finished the bachelor till the end of your first master semester with the expected grad (or better). If your grade is worse, you might be exmatriculated since the expected grade is used in the admission process. You can find the regulations and needed document in the general admission regulations of the HU Berlin (German: Nichtamtliche Lesefassung der Fächerübergreifenden Satzung zur Regelung von Zulassung, Studium und Prüfung der Humboldt-Universität zu Berlin).