don't show 'Documentation for null' in dynamic DOM (IDEA-60302)

xml/dom-impl/src/com/intellij/xml/impl/dom/AbstractDomChildrenDescriptor.java

@@ -115,6 +115,9 @@ public abstract class AbstractDomChildrenDescriptor implements XmlElementDescrip
         @Nullable
         public PsiElement getDeclaration() {
           final PomTarget target = ((CustomDomChildrenDescription)description).getTagNameDescriptor().findDeclaration(finalDomElement);
+          if (target == description) {
+            return childTag;
+          }
           return target == null ? null : PomService.convertToPsi(childTag.getProject(), target);
         }