fix: Handle error when offscreen document already exists

A `try..catch` block has been added to catch errors about the Offscreen
document already existing. This circumstance is not a bug, it's
expected to happen sometimes due to race conditions between
`hasDocument` and the creation step. The code in question is just meant
to create the document if it doesn't exist, so it's OK if we get this
error.

The `hasDocument` call has been removed as well, since it's now
redundant. This function wasn't safe to call anyway; it's an
undocumented private function.

See here for more information:
* https://stackoverflow.com/a/7725839
* https://groups.google.com/a/chromium.org/g/chromium-extensions/c/D5Jg2ukyvUc4

Fixes #25118

app/scripts/app-init.js

@@ -194,18 +194,24 @@ registerInPageContentScript();
  * folder for more details.
  */
 async function createOffscreen() {
-  if (await chrome.offscreen.hasDocument()) {
-    return;
+  try {
+    await chrome.offscreen.createDocument({
+      url: './offscreen.html',
+      reasons: ['IFRAME_SCRIPTING'],
+      justification:
+        'Used for Hardware Wallet and Snaps scripts to communicate with the extension.',
+    });
+    console.debug('Offscreen iframe loaded');
+  } catch (error) {
+    if (
+      !error?.message?.startsWith(
+        'Only a single offscreen document may be created',
+      )
+    ) {
+      throw error;
+    }
+    console.debug('Offscreen document already exists; skipping creation');
   }
-
-  await chrome.offscreen.createDocument({
-    url: './offscreen.html',
-    reasons: ['IFRAME_SCRIPTING'],
-    justification:
-      'Used for Hardware Wallet and Snaps scripts to communicate with the extension.',
-  });
-
-  console.debug('Offscreen iframe loaded');
 }
 
 createOffscreen();