[AccordionSummary] false positive propType warning with disableGeneration

[eahenke]

When AccordionSummary is checking for usage of the deprecated classes.focused prop, the code calls indexOf on classes.focused without checking if that property is defined. If you don't supply classes.focused, this throws Failed prop type: Cannot read property 'indexOf' of undefined.

It appears to have been introduced in this change: 9c48d0b#diff-a3e45615b0516b905db6b26935126e30763f45b8901b2bee960adb5c3be2b997R147

A workaround is to always declare focused as an empty string in the classes prop (note: this does not work in Typescript, since that class was removed from the class keys), but ideally the checker should check if it's defined first.

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

An error is thrown if you use AccordionSummary without explicitly passing classes.focused

Expected Behavior 🤔

No error should be thrown.

Steps to Reproduce 🕹

CodeSandbox: https://codesandbox.io/s/thirsty-ives-bwmqv

Steps:

1. Use ServerStyleSheets with disableGeneration: true option
2. Use AccordionSummary with a classes prop that does not include focused
3. Receive propType warning Failed prop type: Cannot read property 'indexOf' of undefined

Context 🔦

This can break tests that check that components are being passed the correct props (for example, using the prop-types-throw library: https://www.npmjs.com/package/prop-types-throw)

Your Environment 🌎

`npx @material-ui/envinfo`

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  Output from `npx @material-ui/envinfo` goes here.

[eps1lon]

It doesn't look like this bug report has enough info for one of us to reproduce it.

Please provide a CodeSandbox (https://material-ui.com/r/issue-template-latest), a link to a repository on GitHub, or provide a minimal code example that reproduces the problem.

Here are some tips for providing a minimal example: https://stackoverflow.com/help/mcve

[eahenke]

It doesn't look like this bug report has enough info for one of us to reproduce it.

Please provide a CodeSandbox (https://material-ui.com/r/issue-template-latest), a link to a repository on GitHub, or provide a minimal code example that reproduces the problem.

Thanks, updated with a CodeSandbox link and repro steps with some additional context.

[eps1lon]

Fixed in #27385