Incorrect warning about non-default platform/variant from validator

[Nicklas Nordborg]

Setup a project and an array design as follows:

• Create a new project with a default platform+variant
• Create a new array design with the same platform+variant
• Make sure the project is active and the go to the "Overview" tab for the array design and click the "Validate" button.

If the validator options are at their default setting the following warning should be displayed:

Use of non-default platform  	Array design: New array design

which is not correct.

After looking through the code it seems like the validation happens before the variant child node has been loaded. Debug out from the PlatformValidator.postValidate() method indicates that the defaultPlatform, defaultVariant and platform variables are correct, but the variant variable is null. I think it happens because the validation takes place before the variant child node is loaded in the tree. I think that we may actually need a separate loader and validator implementation for the variant node instead of trying to load both the platform and variant in the PlatformLoader class.

[Nicklas Nordborg]

(In [5126]) Fixes #1392: Incorrect warning about non-default platform/variant from validator