#vulnerable code 
                                @Override
    public void add(Widget child) {
        if(child instanceof MaterialImage) {
            child.getElement().getStyle().setProperty("border", "1px solid #e9e9e9");
            child.getElement().getStyle().setProperty("textAlign", "center");
        }

        boolean collapsible = child instanceof MaterialCollapsible;
        if(collapsible) {
            // Since the collapsible is separ
            ((MaterialCollapsible)child).addClearActiveHandler(new ClearActiveHandler() {
                @Override
                public void onClearActive(ClearActiveEvent event) {
                    clearActive();
                }
            });
        }

        if(!(child instanceof ListItem)) {
            // Direct list item not collapsible
            final ListItem listItem = new ListItem();
            if(child instanceof MaterialCollapsible) {
                listItem.getElement().getStyle().setBackgroundColor("transparent");
            }
            if(child instanceof HasWaves) {
                listItem.setWaves(((HasWaves) child).getWaves());
                ((HasWaves) child).setWaves(null);
            }
            listItem.add(child);

            child = listItem;
        }

        // Collapsible's should not be selectable
        final Widget finalChild = child;
        if(!collapsible) {
            // Active click handler
            finalChild.addDomHandler(new ClickHandler() {
                @Override
                public void onClick(ClickEvent event) {
                    clearActive();
                    finalChild.addStyleName("active");
                }
            }, ClickEvent.getType());
        }
        child.getElement().getStyle().setDisplay(Style.Display.BLOCK);
        super.add(child);
    } 
                            #location 27 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Override
    public void add(Widget child) {
        super.add(wrap(child));
    }