Crash when potentialError is null

[Pajn]

🐛 Bug Report

I got this error in my testsuite:

    TypeError: Cannot read property 'message' of null

      at isError (node_modules/jest-jasmine2/build/is_error.js:20:27)

It seems that isError in jest-jasmine2 does not consider null. The typeof === 'object' check is true for null but the code assumes that succeeding that checks makes it possible to read the property message.
https://github.com/facebook/jest/blob/c93e02718480b9f25b0d28f33456ed159bd11b69/packages/jest-jasmine2/src/is_error.js#L15
A clear and concise description of what the bug is.

This seems to happen when null is thrown inside a promise.

[SimenB]

Mind putting together a small repro on e.g. repl.it? Or event better, a PR :)

[Pajn]

It's more work to create a repro than a PR so I will send a PR. I did verify the fix (also checking for null) by modifying the file in node_modules for the affected repo.

[github-actions]

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