Make whitebox API functions more stable

Executing the following code crashes the VM with a segmentation fault. The whitebox API functions should be more stable. In this case a NullPointerException should be thrown.

public class Test {
    private static final WhiteBox WHITE_BOX = WhiteBox.getWhiteBox();

    static public void main(String[] args) {
        WHITE_BOX.isMethodCompiled(null);
    }
}