SWT Combo prevent shift + 9 event

Question

I'm developing an Eclipse RAP application where I have to use a combo box. The user can type a RQL filter in the combo or to select already existing one. The problem comes when the user types a left bracket "(" - SHIFT + 9 combination. This combination traverse the existing filters - started from first to the last for each combination and deleted all the stuff typed before that. I tried with the code below and with TraverseListener, but the event continue to occurs.

combo.addKeyListener(new KeyAdapter() {

      @Override
      public void keyPressed(KeyEvent event) {
        if (event.character == '\u0000') { // used to prevent SHIFT + 9 (left bracket) combination when typing a filter in the combo
          event.doit = false;
        }
      }
    });

Could some one helps me with this? Thanks in advance!