According to the MIDP API at http://java.sun.com/javame/reference/apis/jsr118/javax/microedition/lcdui/TextField.html#setString\(java.lang.String), passing null to TextField.setString() should empty the field. Instead, a NullPointerException is thrown (there is a missing null check in the overloaded method).
http://code.google.com/p/microemu/issues/detail?id=7
Issue has been moved to http://code.google.com/p/microemu/issues/detail?id=7