Firefox WebExtensions API `search.get()` throws an error when invoked from Ubuntu distribution

Readme.md

Prerequisites

Firefox 68.0.1 (64bit) installed from Ubuntu distribution,

Steps to reproduce

1. Copy the files manifest.json and background.js in to a folder
2. Run $ web-ext run
3. In address bar type about:debugging. Temporary extensions 'Test Extension' will be loaded.
4. Click 'Debug'

It gives an error, Error: "An unexpected error occurred"

background.js

console.log("Loading background script");
function retrieved(results) {
  console.log(`There were: ${results.length} search engines retrieved.`);
  for (let searchEngine of results) {
    console.log(JSON.stringify(searchEngine.name));
  }
}

browser.search.get().then(retrieved).catch(function(err) {
    console.log("Error", err);
});

manifest.json

{
    "manifest_version": 2,
    "name": "Test Extension",
    "version": "0.0.1",
    "background": {
        "scripts": [
            "background.js"
        ]
    },
    "permissions": [
        "search"
    ]
}