Fix NullPointerException in Spring interceptor (#4063)

apm-sniffer/apm-sdk-plugin/spring-plugins/core-patch/src/main/java/org/apache/skywalking/apm/plugin/spring/patch/GetPropertyDescriptorsInterceptor.java

@@ -41,7 +41,8 @@ public class GetPropertyDescriptorsInterceptor implements InstanceMethodsAroundI
 
         PropertyDescriptor[] propertyDescriptors = (PropertyDescriptor[])ret;
 
-        if (EnhancedInstance.class.isAssignableFrom(((BeanWrapperImpl)objInst).getRootClass())) {
+        Class<?> rootClass = ((BeanWrapperImpl) objInst).getRootClass();
+        if (rootClass != null && EnhancedInstance.class.isAssignableFrom(rootClass)) {
             List<PropertyDescriptor> newPropertyDescriptors = new ArrayList<PropertyDescriptor>();
             for (PropertyDescriptor descriptor : propertyDescriptors) {
                 if (!"skyWalkingDynamicField".equals(descriptor.getName())) {