chore: prevent NPE in solution annotation processing

Author: triceo

core/src/main/java/ai/timefold/solver/core/impl/domain/solution/descriptor/SolutionDescriptor.java

@@ -83,6 +83,7 @@
 import ai.timefold.solver.core.preview.api.domain.metamodel.PlanningSolutionMetaModel;
 import ai.timefold.solver.core.preview.api.domain.solution.diff.PlanningSolutionDiff;
 
+import org.jspecify.annotations.NonNull;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -395,20 +396,9 @@ Maybe add a getScore() method with a @%s annotation."""
     }
 
     private void processSolutionAnnotations(DescriptorPolicy descriptorPolicy) {
-        var solutionAnnotation = solutionClass.getAnnotation(PlanningSolution.class);
-        var parentSolutionAnnotation =
-                solutionClass.getSuperclass() != null ? solutionClass.getSuperclass().getAnnotation(PlanningSolution.class)
-                        : null;
-        if (solutionAnnotation == null && parentSolutionAnnotation == null) {
-            throw new IllegalStateException(
-                    "The solutionClass (%s) has been specified as a solution in the configuration, but does not have a @%s annotation."
-                            .formatted(solutionClass, PlanningSolution.class.getSimpleName()));
-        }
-        var annotation = solutionAnnotation != null ? solutionAnnotation : parentSolutionAnnotation;
+        var annotation = extractTopMostPlanningSolutionAnnotation();
         autoDiscoverMemberType = annotation.autoDiscoverMemberType();
-        // We accept only the child class cloner
-        var solutionClonerClass =
-                solutionAnnotation != null ? solutionAnnotation.solutionCloner() : PlanningSolution.NullSolutionCloner.class;
+        var solutionClonerClass = annotation.solutionCloner();
         if (solutionClonerClass != PlanningSolution.NullSolutionCloner.class) {
             solutionCloner = ConfigUtils.newInstance(this::toString, "solutionClonerClass", solutionClonerClass);
         }
@@ -417,6 +407,29 @@ private void processSolutionAnnotations(DescriptorPolicy descriptorPolicy) {
                 new LookUpStrategyResolver(descriptorPolicy, lookUpStrategyType);
     }
 
+    private @NonNull PlanningSolution extractTopMostPlanningSolutionAnnotation() {
+        var solutionAnnotation = solutionClass.getAnnotation(PlanningSolution.class);
+        if (solutionAnnotation != null) {
+            return solutionAnnotation;
+        }
+        var solutionSuperclass = solutionClass.getSuperclass(); // Null if interface.
+        if (solutionSuperclass == null) {
+            throw new IllegalStateException("""
+                    The solutionClass (%s) has been specified as a solution in the configuration, \
+                    but does not have a @%s annotation."""
+                    .formatted(solutionClass.getCanonicalName(), PlanningSolution.class.getSimpleName()));
+        }
+        var parentSolutionAnnotation = solutionSuperclass.getAnnotation(PlanningSolution.class);
+        if (parentSolutionAnnotation == null) {
+            throw new IllegalStateException("""
+                    The solutionClass (%s) has been specified as a solution in the configuration, \
+                    but neither it nor its superclass (%s) have a @%s annotation."""
+                    .formatted(solutionClass.getCanonicalName(), solutionSuperclass.getCanonicalName(),
+                            PlanningSolution.class.getSimpleName()));
+        }
+        return parentSolutionAnnotation;
+    }
+
     private void processValueRangeProviderAnnotation(DescriptorPolicy descriptorPolicy, Member member) {
         if (((AnnotatedElement) member).isAnnotationPresent(ValueRangeProvider.class)) {
             var memberAccessor = descriptorPolicy.getMemberAccessorFactory().buildAndCacheMemberAccessor(member,