no-unknown-property: allowedTags.indexOf is not a function

[AriPerkkio]

Hello, no-unknown-property rule seems to crash in certain cases. This issue was spotted by automated CI run - it is not blocking my development or anything. https://github.com/AriPerkkio/eslint-remote-tester/runs/1568977821?check_suite_focus=true

This was the only (new) rule crashing in this plugin's weekly test. 👍

"eslint-plugin-react": "^7.21.5",

  "rules": {
    "react/no-unknown-property": "error"
  }

Minimal repro:

<div hasOwnProperty="should not be allowed tag" />

TypeError: allowedTags.indexOf is not a function
Occurred while linting <text>:187
    at JSXAttribute (/home/<removed>/eslint-remote-tester/ci/node_modules/eslint-plugin-react/lib/rules/no-unknown-property.js:262:87)```

Crash reports from real projects

Rule: no-unknown-property

• Message: allowedTags.indexOf is not a function Occurred while linting <text>:187
• Path: yzw7489757/react-code-learn/packages/react-dom/src/__tests__/ReactServerRendering-test.js
• Link

      let html;
      expect(
        () =>
          (html = ReactDOMServer.renderToString(
            <div hasOwnProperty="poison">
              <span unknown="test" />
            </div>,
          )),
      ).toWarnDev(['React does not recognize the `hasOwnProperty` prop']);
      expect(html).toContain('<span unknown="test">');

TypeError: allowedTags.indexOf is not a function
Occurred while linting <text>:187
    at JSXAttribute (/home/<removed>/eslint-remote-tester/ci/node_modules/eslint-plugin-react/lib/rules/no-unknown-property.js:262:87)
    at /home/<removed>/eslint-remote-tester/ci/node_modules/eslint-remote-tester/node_modules/eslint/lib/linter/safe-emitter.js:45:58
    at Array.forEach (<anonymous>)
    at Object.emit(/home/<removed>/eslint-remote-tester/ci/node_modules/eslint-remote-tester/node_modules/eslint/lib/linter/safe-emitter.js:45:38)
    at NodeEventGenerator.applySelector(/home/<removed>/eslint-remote-tester/ci/node_modules/eslint-remote-tester/node_modules/eslint/lib/linter/node-event-generator.js:254:26)
    at NodeEventGenerator.applySelectors(/home/<removed>/eslint-remote-tester/ci/node_modules/eslint-remote-tester/node_modules/eslint/lib/linter/node-event-generator.js:283:22)
    at NodeEventGenerator.enterNode(/home/<removed>/eslint-remote-tester/ci/node_modules/eslint-remote-tester/node_modules/eslint/lib/linter/node-event-generator.js:297:14)
    at CodePathAnalyzer.enterNode (/home/<removed>/eslint-remote-tester/ci/node_modules/eslint-remote-tester/node_modules/eslint/lib/linter/code-path-analysis/code-path-analyzer.js:711:23)
    at /home/<removed>/eslint-remote-tester/ci/node_modules/eslint-remote-tester/node_modules/eslint/lib/linter/linter.js:952:32
    at Array.forEach (<anonymous>)