Custom Widget causes clientside exception

Vaadin 6 / 7 / 8
1

Ich created a very simple Widget that extends VTextField. All I want to do, is restrict the characters that can be entered on the client side and then study how vaadin 7 shared state and RPC works.

I followed this tutorial to create the widget and the component: https://vaadin.com/wiki/-/wiki/Main/Creating%20a%20simple%20component
I also attached a ClickHandler that clears the textfield the first time it is clicked.

When I click the special text-field the following happens:

  • The first click causes this exception.

  • The second click clears the field, as expected.
  • The third click or leaving the field causes the exception again.

I must be doing something wrong. I started the app in superDevMode to get the exception with the stack trace.