fix: don't crash if target/user is undefined

Error: TypeError: Cannot read properties of undefined (reading 'displayname')
File: Flags.notify (/usr/src/app/src/flags.js:812:89)

src/flags.js

@@ -807,9 +807,10 @@ Flags.notify = async function (flagObj, uid, notifySelf = false) {
 		});
 		uids = uids.concat(modUids[0]);
 	} else if (flagObj.type === 'user') {
+		const targetDisplayname = flagObj.target && flagObj.target.user ? flagObj.target.user.displayname : '[[global:guest]]';
 		notifObj = await notifications.create({
 			type: 'new-user-flag',
-			bodyShort: `[[notifications:user_flagged_user, ${displayname}, ${flagObj.target.user.displayname}]]`,
+			bodyShort: `[[notifications:user_flagged_user, ${displayname}, ${targetDisplayname}]]`,
 			bodyLong: await plugins.hooks.fire('filter:parse.raw', String(flagObj.description || '')),
 			path: `/flags/${flagObj.flagId}`,
 			nid: `flag:user:${flagObj.targetId}`,