Expression has nothing to do with JS to Bubble. (Expression basically combines the functionality of Run JS and JS to Bubble into one element for cases where you can just run the script passively. But you can make Expression respond in an event-like manner: Set its script time be empty-ish by default — e.g., // code is over in conditions — and then change the code based on some condition, causing the script to change and, as a result, run. This is useful for example when you must wait for the page to be fully loaded until executing the script [e.g., because it relies on some external package]…)
That being said, what the error message means is that someone is trying to add an event listener to something that does not exist.
That “someone”, my friend, is you. 
Your code looks for an input with ID myInput and then attempts to add a listener to it.
However, if “myInput” does not exist in the page, your variable “input” will be null. (Because input is the result of getElementByID… and your target (myInput) does not exist in the page until the popup is displayed. So, instead of getting a handle to the desired element, you get nothing [null]… and you can’t put an event listener on nothing.)
So either move Expression into the popup (may or may not work…test it) OR, take note of my first paragraph. (Set the Expression code to be placeholder, but in the conditionals tab, change expression’s code to your desired code WHEN the popup is visible and element “myInput” actually exists.)
The point is that your Expression’s code (Which runs now on page load) will error when myInput doesn’t exist. And your code will not execute automatically once myInput DOES exist.
So you must force Expression to update (which you can do by changing its code [perhaps it’s called script?] parameter in the conditions tab.
Aside: I had struggled with a similar problem and the author of Expression kindly gave me this same tip. That’s the only reason I know the answer.
While the Toolbox plugins are very clever, they are on the very edge of being “plugins that do nothing” as I’m fond of saying. They are not very Bubbly and deserve a rethink. Expression for example is something like 12 lines (or less) of code and has no API beyond the script input. 
