"Cannot read property '__error' of null" for a pre-configured route returning 500

[travikk]

Current behavior:

I have a route configured in the following way:

cy.route({
         delay: 1000,
         status: 500,
         method: 'POST',
         url: `/sillyPath/blablabla/`,
         response: {},
         headers: {
            'Content-Type': 'application/grpc-web+proto',
         },
      })

When the application makes request, it obviously fails and I have a handler in the code that deals with that.
For some reason, Cypress runner sees a problem and throws the following error:

TypeError: The following error originated from your application code, not from Cypress.

  > Cannot read property '__error' of null

When Cypress detects uncaught errors originating from your application it will automatically fail the current test.

This behavior is configurable, and you can choose to turn this off by listening to the `uncaught:exception` event.
    at XMLHttpRequest._getFixtureError (http://localhost:3000/__cypress/runner/cypress_runner.js:163165:22)
    at onLoadFn (http://localhost:3000/__cypress/runner/cypress_runner.js:162015:27)
    at XMLHttpRequest.<anonymous> (http://localhost:3000/__cypress/runner/cypress_runner.js:161999:25)
From previous event:

That line, maps to the following code in cypress_runner.js:

{
    key: "_getFixtureError",
    value: function _getFixtureError() {
      var body = this.response && this.response.body;
      var err = body.__error; // this causes issue as body is null !

      if (body && err) {
        return err;
      }
    }
  }

Looks like that code should perhaps check if the response.body is null first before trying to access __error?
The net result is that my test is marked as failed, which is incorrect.

Desired behavior:

When I preconfigure a route to return an error, cypress handles it without failing the test.

Versions

Cypress: 3.7.0
OS: Windows/Linux

[jennifer-shehane]

Duplicate of #7518 - this will be fixed in our next release.