Don't initialize the base class twice

[rozyczko]

Description

Base QWidget class was being initialized twice leading to the Generic Scattering Calculator throwing on Draw.

Fixes #2818

How Has This Been Tested?

Local win10 tests

Review Checklist:

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Documentation (check at least one)

• There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)

Licencing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)

[wpotrzebowski]

Tested on Mac and generally it seems to work fine, however, I also had a few unexpected crashes... At the moment I cannot find a reproducible path. (It may also be unrelated to this PR).

[butlerpd]

This fixes the error reported. clicking draw no longer throws a traceback error saying that you cannot initialize the widget twice.

Now that we can in fact see the drawing rendering there is some flakiness with those drawing randomly disappearing and even once crashing while playing with the 3D graphics. That however is clearly a different and deeper issue which has not been spotted because heretofore one could not even get the drawing up.

We should try to document the flakiness better and create an issue(s) that need addressing