Error: Type 'undefined' is not assignable to type 'RecordProxy<{}>'.

[kmjennison]

I'm seeing this error when running yarn dev:

ERROR in /my/path/offline-examples/relay/nextjs/mutations/AddTodoMutation.ts
64:37 Argument of type 'RecordProxy<{}> | null | undefined' is not assignable to parameter of type 'RecordProxy<{}>'.
  Type 'undefined' is not assignable to type 'RecordProxy<{}>'.
    62 | 
    63 |   // Insert the new todo into the Todo List connection
  > 64 |   ConnectionHandler.insertEdgeAfter(conn, newEdge);
       |                                     ^
    65 | }
    66 | 
    67 | function commit(environment: RelayModernEnvironment, text: string, user: any) {

This error is happening with cleared storage on localhost using the packages updated in #16.

I think it's caused by this TypeScript bug, because downgrading to 3.0.1 fixes it:
microsoft/TypeScript#29505

PR incoming to downgrade TypeScript.