Handle when no iscomplete correctly in callback

dash_uploader/callbacks.py

@@ -2,6 +2,7 @@
 from pathlib import Path
 
 from dash import __version__ as dashversion
+from dash.exceptions import PreventUpdate
 from dash.dependencies import Input, State
 
 import dash_uploader.settings as settings
@@ -28,7 +29,7 @@ def create_dash_callback(callback, settings):  # pylint: disable=redefined-outer
 
     def wrapper(iscompleted, filenames, upload_id):
         if not iscompleted:
-            return
+            raise PreventUpdate()
 
         out = []
         if filenames is not None:

docs/CONTRIBUTING.md

@@ -23,6 +23,10 @@ Maybe you already have an idea. If not, see if there are any open [issues](https
 - Clone this repository. Change current directory to project root.
 - Install [npm and Node.js](https://nodejs.org) for building JS.
 - Install the JS dependencies by running `npm install` on the project root. This will create `node_modules` directory.
+- Build the NPM modules
+```
+ npm run build
+```
 - Create python virtual environment and activate it
 - `pip install` this package in editable state with the `[dev]` flag.
 ```