Configure correct app dir when building /_error fallback in dev (#75354)

Author: eps1lon

packages/next/src/build/webpack-config.ts

@@ -304,13 +304,13 @@ export default async function getBaseWebpackConfig(
     dev?: boolean
     entrypoints: webpack.EntryObject
     isDevFallback?: boolean
-    pagesDir?: string
+    pagesDir: string | undefined
     reactProductionProfiling?: boolean
     rewrites: CustomRoutes['rewrites']
     originalRewrites: CustomRoutes['rewrites'] | undefined
     originalRedirects: CustomRoutes['redirects'] | undefined
     runWebpackSpan: Span
-    appDir?: string
+    appDir: string | undefined
     middlewareMatchers?: MiddlewareMatcher[]
     noMangling?: boolean
     jsConfig: any

packages/next/src/server/dev/hot-reloader-webpack.ts

@@ -692,6 +692,7 @@ export default class HotReloaderWebpack implements NextJsHotReloaderInterface {
       config: this.config,
       buildId: this.buildId,
       encryptionKey: this.encryptionKey,
+      appDir: this.appDir,
       pagesDir: this.pagesDir,
       rewrites: {
         beforeFiles: [],