Handle potentially undefined `event.extra` field

This issue was noticed after a TypeScript update.

src/sidebar/util/sentry.js

@@ -88,6 +88,9 @@ export function init(config) {
       try {
         const originalErr = hint && hint.originalException;
         if (originalErr instanceof Event) {
+          if (!event.extra) {
+            event.extra = {};
+          }
           Object.assign(event.extra, {
             type: originalErr.type,
             // @ts-ignore - `detail` is a property of certain event types.

src/sidebar/util/test/sentry-test.js

@@ -164,7 +164,7 @@ describe('sidebar/util/sentry', () => {
     it('extracts metadata from thrown `Event`s', () => {
       sentry.init({ dsn: 'test-dsn' });
       const beforeSend = getBeforeSendHook();
-      const event = { extra: {} };
+      const event = {};
 
       beforeSend(event, {
         originalException: new CustomEvent('unexpectedevent', {
@@ -184,7 +184,7 @@ describe('sidebar/util/sentry', () => {
     it('ignores errors serializing non-Error exception values', () => {
       sentry.init({ dsn: 'test-dsn' });
       const beforeSend = getBeforeSendHook();
-      const event = { extra: {} };
+      const event = {};
       const originalException = new CustomEvent('unexpectedevent');
       Object.defineProperty(originalException, 'detail', {
         get: () => {