Add a nil check to prevent a crash

[amgleitman]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

We've seen a crash downstream where -[NSString stringByReplacingCharactersInRange:withString:] receives a nil value as the replacement string. This is not good, since we expect that argument to be non-null.

We believe that a cause of this is that -[RCTUITextField textView:shouldChangeTextInRange:replacementString:] is being called with nil as the replacement string. (This is legal, as per Apple's documentation.) Right now, the only check that this delegate method does is enforcing the maxLength parameter if it exists, and changes in attributes shouldn't affect the length of the string.

Changelog

[macOS] [Fixed] - -[RCTUITextField textView:shouldChangeTextInRange:replacementString:] no longer crashes when we pass in a nil replacement string

[harrieshin]

should the predicted text be empty string if the replacement string is nil or backedTextInputView.attributedText.string?

[amgleitman]

should the predicted text be empty string if the replacement string is nil or backedTextInputView.attributedText.string?

Passing a replacement string of nil means that we're not changing the text, just attributes (see the link to Apple's documentation above), so I think we should be good in leaving _predictedText unchanged in that case.