SHAPE: Fix undefined behavior resulting in strange lock-ups

SHAPE: Fix undefined behavior resulting in strange lock-ups

This commit fixes intermittent freezes of 'subscriber' dialog. These freezes
were caused by invoking QWidget::update() directly from a non-GUI thread.
This is undefined behavior and leads to many strange lock-ups and even SEGFAULTS.

In order to fix it we use QMetaObject::invokeMethod(), which is thread-safe.