TypeError when trying to use element picker on plaintext resource

[vtriolet]

Prerequisites

• I verified that this is not a filter issue (MUST be reported at filter issue tracker)
• This is not a support issue or a question
• I performed a cursory search of the issue tracker to avoid opening a duplicate issue
• The issue is not present after wholly disabling uBlock Origin ("uBO") in the browser
• I checked the documentation to understand that the issue I report is not a normal behavior

I tried to reproduce the issue when...

• uBO is the only extension
• uBO with default lists/settings
• using a new, unmodified browser profile

Description

A TypeError is triggered when you try to use the element picker on a plaintext resource.

Firefox:

TypeError: vAPI is undefined epicker.js:33:19

Chrome:

Uncaught (in promise) TypeError: Cannot read property 'randomToken' of undefined
    at VM11 epicker.js:33
    at VM11 epicker.js:1294

A specific URL where the issue occurs

https://www.google.com/robots.txt

Steps to Reproduce

1. Navigate to https://www.google.com/robots.txt
2. Right-click on the page and select 'Block element...' to trigger a TypeError

Expected behavior

I'm not sure if the element picker can work with plaintext resources, but it seems desirable to at least avoid the TypeError.

Actual behavior

A TypeError is triggered.

uBlock Origin version

1.37.2

Browser name and version

Firefox 90.0.2, Chrome 92.0.4515.107

Operating System and version

Arch Linux