NestedParameter.AssociatedParameter seems to reference the wrong ParameterBase

[lxatstariongroup]

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Description

Original message from Stephan Jahnke who found the issue:

FYI, I found a solution/workaround to this problem via checking whether ElementUsage.ParameterOverride List already contains a ParameterOverride for the selected ParameterType.

My first idea to check 
If(nestedParameter.AssociatedParameter is ParameterOverride) always returned false, as AssociatedParameter always seems to be of type Parameter, even if an ParameterOverride already exists.

According to the docs this could also be a ParameterSubscription, or ParameterOverrride.

Please check and fix accordingly.