You must add the following code in the costructor
getWindow().addFlags(WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE | WindowManager.LayoutParams.FLAG_ALT_FOCUSABLE_IM);
Often you will want to have a Dialog display on top of the current input method, because there is no reason for it to accept text. You can do this by setting the WindowManager.LayoutParams.FLAG_ALT_FOCUSABLE_IM window flag (assuming your Dialog takes input focus, as it the default) with the following code:
getWindow().setFlags(WindowManager.LayoutParams.FLAG_ALT_FOCUSABLE_IM,
WindowManager.LayoutParams.FLAG_ALT_FOCUSABLE_IM);
Also found here