Requesting an image with a non-existing image variation causes a NullPointerException in STKImageGenerator.generate.

Lines 59 and 60 of STKImageGenerator are:

final ImageOperationProvidingVariation variation = parameter.getImageVariation();
final ImageOperation<ParameterProvider<NodeData>> imageOperation = variation.getImageOperation();

For a request to an image with a non-existing image variation parameter.getImageVariation() will return null, which will be assigned to the variation variable. This causes a NullPointerException on the next line.