Unchecked Runtime. lastError When Using Chrome Api

This error is not important in this case, but I'll explain it and how to get rid of it.

What is this error?

Chrome APIs are mostly asynchronous: you have a callback that is called when the operation completes.

In case of chrome.fileSystem.chooseEntry, the chosen entry (or entries) will be passed to a callback:

chrome.fileSystem.chooseEntry(
  {/* options */},
  function(entry) {
    // This is the callback for the API call; you can do something with entry
  }
);

However, the API is not guaranteed to yield a result. For example, as in your case, the user may refuse to provide access by clicking "Cancel". Then there is no entry to work with, and you might want some explanation as to why that happened. How to raise an error without polluting all callbacks with an extra "error" argument?

Usually, Chrome deals with this by setting a global variable, chrome.runtime.lastError, at the time the callback is called. It is uniform across Chrome async APIs to use this instead of an error argument. In fact, quoting the chrome.fileSystem docs:

All failures are notified via chrome.runtime.lastError.

• If everything is all right, it will be undefined.
• If there is a problem, it will be non-empty, and chrome.runtime.lastError.message will explain what's wrong.

However, some callbacks did not check for this error variable. This may indicate a programming error, and Chrome added checks that chrome.runtime.lastError is actually checked (evaluated) in a callback. If not, it considers this to be an unhandled exception, and throws this error.