Widget annotations: do not crash if `Parent` is not a dictionary

during field name construction (issue 8143)

src/core/annotation.js

@@ -671,6 +671,12 @@ var WidgetAnnotation = (function WidgetAnnotationClosure() {
       var loopDict = dict;
       while (loopDict.has('Parent')) {
         loopDict = loopDict.get('Parent');
+        if (!isDict(loopDict)) {
+          // Even though it is not allowed according to the PDF specification,
+          // bad PDF generators may provide a `Parent` entry that is not a
+          // dictionary, but `null` for example (issue 8143).
+          break;
+        }
 
         if (loopDict.has('T')) {
           fieldName.unshift(stringToPDFString(loopDict.get('T')));

test/unit/annotation_spec.js

@@ -801,6 +801,28 @@ describe('annotation', function() {
 
       expect(data.fieldName).toEqual('foo.bar.baz');
     });
+
+    it('should construct the field name if a parent is not a dictionary ' +
+       '(issue 8143)', function() {
+      var parentDict = new Dict();
+      parentDict.set('Parent', null);
+      parentDict.set('T', 'foo');
+
+      widgetDict.set('Parent', parentDict);
+      widgetDict.set('T', 'bar');
+
+      var widgetRef = new Ref(22, 0);
+      var xref = new XRefMock([
+        { ref: widgetRef, data: widgetDict, }
+      ]);
+
+      var annotation = annotationFactory.create(xref, widgetRef,
+                                                pdfManagerMock, idFactoryMock);
+      var data = annotation.data;
+      expect(data.annotationType).toEqual(AnnotationType.WIDGET);
+
+      expect(data.fieldName).toEqual('foo.bar');
+    });
   });
 
   describe('TextWidgetAnnotation', function() {