Steps to repro

1. Launch Brackets and load the brackets source
2. Create a new, untitled document
3. Enter some JavaScript. I used:

function blah() {
    DocumentManager.getCurrentDocument();
}

1. Put your cursor in the name of a function that is defined in another file (like getCurrentDocument) and press Cmd-E.

Results
Inline editor is not opened, and an error is output to the console:

Uncaught TypeError: Cannot call method 'getCursor' of null