fix(BpmnTreeWalker): do not fail if no di

lib/BpmnTreeWalker.js

@@ -14,7 +14,7 @@ function BpmnTraverser(visitor) {
   }
   
   function is(element, type) {
-    return element.__isInstanceOf(type);
+    return element.__instanceOf(type);
   }
 
   function visit(element, di, ctx) {
@@ -82,7 +82,14 @@ function BpmnTraverser(visitor) {
     }
 
     if (!diagram) {
-      diagram = diagrams[0];
+      if (diagrams && diagrams.length) {
+        diagram = diagrams[0];
+      }
+    }
+
+    // no diagram -> nothing to import
+    if (!diagram) {
+      return;
     }
 
     var rootElement = diagram.plane.bpmnElement;

test/spec/browser/ImporterSpec.js

@@ -36,4 +36,15 @@ describe('Importer', function() {
       diagram.importDefinitions(result, done);
     });
   });
+
+  it('should import empty definitions', function(done) {
+
+    var xml = fs.readFileSync('test/fixtures/bpmn/empty-definitions.bpmn', 'utf8');
+
+    read(xml, function(err, result) {
+      var diagram = new Diagram(container);
+
+      diagram.importDefinitions(result, done);
+    });
+  });
 });