Enable strict mode for extension build

ext/src/lib/loadSender.ts

@@ -1,5 +1,6 @@
 "use strict";
 
+import logger from "./logger";
 import { stringify } from "./utils";
 
 import { ReceiverSelection
@@ -27,6 +28,11 @@ export default async function loadSender (opts: LoadSenderOptions) {
     switch (opts.selection.mediaType) {
         case ReceiverSelectorMediaType.App: {
             const shim = ShimManager.getShim(opts.tabId, opts.frameId);
+            if (!shim) {
+                throw logger.error(`Shim not found at tabId ${
+                        opts.tabId} / frameId ${opts.frameId}`)
+            }
+
             shim.contentPort.postMessage({
                 subject: "shim:/launchApp"
               , data: { receiver: opts.selection.receiver }