Now we are ready to test the web part. Save the web part file and type the following commands to the Command Prompt. If you are developing your web parts and have a feeling that not all of your modifications are present on the workbench, it is a good idea to use gulp clean and start fresh.
gulp clean
gulp serve
After you have tested that everything works as expected on the local SharePoint Workbench, open a hosted SharePoint Workbench from your SharePoint site. This is found under https://{site}/_layouts/workbench.aspx.
One possible error you might find is that the web part can't find the list. This is what it will look like:
You can easily test this by modifying the URL of the spHttpClient.post call. Alternatively, you can create the feedback list so that the name of the URL field is Url instead of URL. This will give you an error message that is less clear.
When troubleshooting this kind of situation, you need more information...