Avoid NPE for DerivedAnnotations not backed by an EStructuralFeature

Issue-Ref: 4138
Issue-Url: #4138

Signed-off-by: Simon Barner <barner@fortiss.org>

org.fortiss.tooling.base/src/org/fortiss/tooling/base/annotation/valueprovider/.ratings

 AnnotationInstSpec.java b4f2ed47a8984e751e04049de5bdb3cad2c0a933 GREEN
 DerivedAnnotationValueProviderBase.java afedd21d3469127bbb20adb34c191b5c9c980f6c GREEN
-EStructuralFeatureDescriptor.java 416e3cd900151e538922539b0b466908a1cf4639 GREEN
+EStructuralFeatureDescriptor.java f27e01608d167f4e02bcfe61a8eafa823675fbee YELLOW
 EStructuralFeatureValueProviderBase.java 287facbbce47c16d892bae82a214f64ceeef2263 GREEN
 IAnnotationValueProvider.java 08e0e5f66dc97865e9ac03e1ac646af332845e14 GREEN
 ValueProviderBase.java e4e866840845346ec99a4304048f5327c4890996 GREEN

org.fortiss.tooling.base/src/org/fortiss/tooling/base/annotation/valueprovider/EStructuralFeatureDescriptor.java

@@ -159,7 +159,8 @@ public class EStructuralFeatureDescriptor {
 	 */
 	public EClassifier getEType(IAnnotatedSpecification specification) {
 		try {
-			return getEStructuralFeature(specification).getEType();
+			EStructuralFeature esf = getEStructuralFeature(specification);
+			return esf != null ? esf.getEType() : null;
 		} catch(Exception e) {
 			return null;
 		}