title: Angular 9 - NGCC fails with an unhandled exception
body: <p>Building the application after having upgraded dependencies to Angular 9 (and having performed the necessary code changes) throws an error:</p>
  <blockquote>
  <p>Compiling @angular/animations : es2015 as esm2015   Compiling
  @angular/animations : es2015 as esm2015   Compiling @angular/core :
  es2015 as esm2015   Compiling @angular/core : es2015 as esm2015
  Compiling @angular/core : es2015 as esm2015
  <strong>Error</strong>: Error on worker #5: TypeError: Cannot read property 'fileName' of null</p>
  </blockquote>
  <p>It then goes on to throw the below error:</p>
  <blockquote>
  <p>Compiling @angular/core : es2015 as esm2015   Compiling
  @angular/compiler/testing : es2015 as esm2015   Compiling
  @angular/core : es2015 as esm2015
  <strong>Error</strong>: Tried to write node_modules/@angular/core/core.d.ts.__ivy_ngcc_bak with an ngcc back
  up file   but it already exists so not writing, nor backing up,
  node_modules/@angular/core/core.d.ts.</p>
  </blockquote>
  <p>This error may be because two or more entry-points overlap and ngcc has been asked to process some files more than once.
  You should check other entry-points in this package and set up a configuration to ignore any that you are not using.</p>
  <blockquote>
  <p>Compiling @angular/core : es2015 as esm2015   An unhandled exception
  occurred: NGCC failed.</p>
  </blockquote>