The bug was caused by a coincidence of four problems:

1) KoAspectButton never did initialization of the icon in the constructor,
   it waited for someone to call setKeepAspectRatio() to set up the icon

2) Constructor of KisWdgBlur never initialized m_halfSizeLink variable
   and the button.

3) [not fixed] KisWdgBlur::setConfiguration() (and a lot of other filters) just skip
   initializing the GUI control in case the property is absent in the
   configuration. Ideally, it should fall back to the default value
   instead.

4) KisBlurFilter::defaultConfiguration() didn't add a property for
   "lockAspect", so the control has never been initialized unless the
   user clicked on it.

BUG:438269