I’m trying to use browser.runtime.sendMessage from background.js to a content script, but I keep having the error:
Uncaught (in promise) Error: Could not establish connection. Receiving end does not exist. <anonymous> moz-extension://e188c1d0-70e8-4b82-8e62-287a34ac151c/background.js:1

Here’s background.js:

browser.runtime.sendMessage({hi:'hi'})

Here is content_scripts/sometest.js:

browser.runtime.onMessage(message => {
  console.log(message)
});

I even have a button in the pop up that should be sending a message whenever I click it. But it stills produces the same error, even if I click outside about:debugging or a mozilla page (which theoretically you can’t inject content scripts). Here is choose_beast.js:

const storeone = document.querySelector("#storeone");

console.log(storeone) //this gets logged

storeone.addEventListener("click", () => {
  console.log("clicked"); // also this gets logged in the console!
  browser.runtime.sendMessage({action:"send_oi"}) //this produces the same error
});

And finally, here is my manifest.json

{

  "description": "Adds a browser action icon to the toolbar. Click the button to choose a beast. The active tab's body content is then replaced with a picture of the chosen beast. See https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Examples#beastify",
  "manifest_version": 2,
  "name": "Beastify",
  "version": "1.0",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/beastify",
  "icons": {
    "48": "icons/beasts-48.png"
  },

  "permissions": [
    "activeTab",
    "webRequest",
    "webRequestBlocking"
  ],

  "content_scripts": [
    {
        "matches": ["<all_urls>"],
        "js": ["content_scripts/sometest.js"]
    }
  ],

  "background": {
    "scripts": ["background.js"],
    "persistent": false
  },

  "browser_action": {
    "default_icon": "icons/beasts-32.png",
    "theme_icons": [{
        "light": "icons/beasts-32-light.png",
        "dark": "icons/beasts-32.png",
        "size": 32
    }],
    "default_title": "Beastify",
    "default_popup": "popup/choose_beast.html"
  },

  "web_accessible_resources": [
    "beasts/*.jpg",
    "redirect.html"
  ]

}