Fix typeof check for notify in core

[benjaminreid]

Description

Fixes #1105, a typeof check which was evaluating to true even when notify would be undefined.

Checklist

• The version field in package.json is incremented following semantic versioning
• The box that allows repo maintainers to update this PR is checked
• I tested locally to make sure this feature/fix works
• This PR passes the Circle CI checks

[Adamj1232]

Nice, thank you!