Yes, it is possible to apply for the programme while you are still in your Bachelor’s. Therefore, you have to verify that you need less than or equal to 30 ECTS to finish your Bachelor’s. Additionally, you have to state the missing courses and your expected Bachelor’s grade. If you are accepted to the programme, you have until the end of your first Master’s semester to prove that you finished the Bachelor’s degree with the expected grade (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 required document in the general admission regulations of HU Berlin (German: Nichtamtliche Lesefassung der Fächerübergreifenden Satzung zur Regelung von Zulassung, Studium und Prüfung der Humboldt-Universität zu Berlin).