We rely on the onload callback of the script element. (See PageEditorBridge#40).

But it turns out that although the onload is called after the script is downloaded by the browser and inserted to DOM, it takes several milliseconds for the necessary functions to be loaded. See this fiddle: https://jsfiddle.net/RobSis/bgk62y9t/10/

Currently, we delay the call by 200 milliseconds, which explains why it happens only sometimes. Something more robust will be needed, either a wait loop or some kind of callback from the PageEditor script.