common-cannot-read-properties-of-null-reading-addeventlistener.md

title	description	type	page_title	slug	position	tags	ticketid	res_type
Cannot read properties of null (reading addEventListener)	How to resolve JavaScript JSInterop error about addEventListener in Blazor WebAssembly applications.	troubleshooting	Cannot read properties of null (reading addEventListener)	common-kb-cannot-read-properties-of-null-reading-addeventlistener		error, update, WASM, WebAssembly	1545248, 1550215, 1550371, 1552007	kb

Environment

Product	UI for Blazor
Blazor application type	WebAssembly

Description

JSInterop (JavaScript) error in the browser devtools console:

Cannot read properties of null (reading 'addEventListener')

Steps to Reproduce

The error can occur in WebAssembly (WASM) projects after updating UI for Blazor to a new version.

Error Message

````JS Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100] Unhandled exception rendering component: Cannot read properties of null (reading 'addEventListener') TypeError: Cannot read properties of null (reading 'addEventListener') ````

````JS Microsoft.JSInterop.JSException: Cannot read properties of null (reading 'addEventListener') TypeError: Cannot read properties of null (reading 'addEventListener') ````

Possible Cause

The error indicates that the app is still using the old version of our telerik-blazor.js file.

Solution

To resolve the error:

• (if using CDN) Update the telerik-blazor.js file URL to the correct version.
• (if using static assets) Clear the browser cache and add a cache buster for the Telerik assets.
• (if using a local JS file) Replace the telerik-blazor.js file with the new version.

Notes

The Blazor framework should normally take care of caching issues with static assets, but they might still occur sometimes.

See Also

• Telerik UI for Blazor version upgrade steps
• Troubleshooting JavaScript Errors
• Fix styling issues after component version update