Version 7.0.1
The full stack trace can be found below.

The issue occurs when I begin to add the key to this code:

      this.state.array.map((arrayItem, index) => (
        <div>
          <button onClick={() => this.delete(index)}>Delete this thing</button>
        </div>
      ))

If I begin to enter the key attribute:

      this.state.array.map((arrayItem, index) => (
        <div key>
          <button onClick={() => this.delete(index)}>Delete this thing</button>
        </div>
      ))

An error in the console is triggered, although I wasn't finished writing the new line.

The issue appears to be on this line, and I suspect it could be corrected altering the line to be if (!value || value.type !== 'JSXExpressionContainer') {, but I'm not sure about the implication of making that change.