Check that the progressCallback is defined before calling it

src/api.js

@@ -699,10 +699,12 @@ var WorkerTransport = (function WorkerTransportClosure() {
       }, this);
 
       messageHandler.on('DocProgress', function transportDocProgress(data) {
-        this.progressCallback({
-          loaded: data.loaded,
-          total: data.total
-        });
+        if (this.progressCallback) {
+          this.progressCallback({
+            loaded: data.loaded,
+            total: data.total
+          });
+        }
       }, this);
 
       messageHandler.on('DocError', function transportDocError(data) {