TypeError: null: type 'JSNull' is not a subtype of type 'String'

TypeError: null: type 'JSNull' is not a subtype of type 'String'

at Object.wrapException (http://localhost:63795/dwds/src/injected/client.js:1249:43)
at Object._asString (http://localhost:63795/dwds/src/injected/client.js:2840:15)
at main__closure7.call$1 (http://localhost:63795/dwds/src/injected/client.js:26677:54)
at _EventStreamSubscription_closure.call$1 (http://localhost:63795/dwds/src/injected/client.js:26040:26)
at StaticClosure._rootRunUnary (http://localhost:63795/dwds/src/injected/client.js:4382:16)
at _CustomZone.runUnary$2$2 (http://localhost:63795/dwds/src/injected/client.js:14273:39)
at _CustomZone.runUnaryGuarded$1$2 (http://localhost:63795/dwds/src/injected/client.js:14220:14)
at _CustomZone_bindUnaryCallbackGuarded_closure.call$1 (http://localhost:63795/dwds/src/injected/client.js:14407:25)
at _callDartFunctionFast1 (http://localhost:63795/dwds/src/injected/client.js:7680:25)
at http://localhost:63795/dwds/src/injected/client.js:7656:18

Start by reproducing the error from the reported URL and inspect the stack trace in dwds/src/injected/client.js, especially _asString and main__closure7. Trace which event supplies null where a String is expected, then verify that the same interaction no longer produces the TypeError.

Written by the indexing model from the issue text.