fix(@ngtools/webpack): fix for #7615

Check if this.done is defined before delaying file access

Author: rezonant

packages/@ngtools/webpack/src/plugin.ts

@@ -390,9 +390,9 @@ export class AotPlugin implements Tapable {
       // Wait for the plugin to be done when requesting `.ts` files directly (entry points), or
       // when the issuer is a `.ts` file.
       compiler.resolvers.normal.plugin('before-resolve', (request: any, cb: () => void) => {
-        if (request.request.endsWith('.ts')
+        if (this.done && request.request.endsWith('.ts')
           || (request.context.issuer && request.context.issuer.endsWith('.ts'))) {
-          this.done!.then(() => cb(), () => cb());
+          this.done.then(() => cb(), () => cb());
         } else {
           cb();
         }