Form Fields Disappear all of a sudden


I have this problem which is bothering me for a while now. My vaadin form fields disappear from the form after some time. As you can see from the images. I have no idea what is happening. As you can see I have uploaded couple of images. First one is the form the way it should appear and initially when the portlet is loaded the form gets displayed perfectly. After few refreshes, the form fields are not rendered at all. as you can see in second image.
Sometimes, some fields are rendered and others not, as you can see in third image.

Here’s the class that does this:

public class SearchForm extends Form implements Handler{