Buidling the application after having upgraded dependcies to Angular 9 (and having performed necessary code changes) throws an error:  

&gt; Compiling @angular/animations : es2015 as esm2015   Compiling
&gt; @angular/animations : es2015 as esm2015   Compiling @angular/core :
&gt; es2015 as esm2015   Compiling @angular/core : es2015 as esm2015  
&gt; Compiling @angular/core : es2015 as esm2015   
&gt; **Error**: Error on worker #5: TypeError: Cannot read property &#39;fileName&#39; of null

It then goes on to throw the below error:  

&gt; Compiling @angular/core : es2015 as esm2015   Compiling
&gt; @angular/compiler/testing : es2015 as esm2015   Compiling
&gt; @angular/core : es2015 as esm2015  
&gt; **Error**: Tried to write node_modules/@angular/core/core.d.ts.__ivy_ngcc_bak with an ngcc back
&gt; up file   but it already exists so not writing, nor backing up,
&gt; node_modules/@angular/core/core.d.ts.

 
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 config to ignore any that you are not using.  

&gt; Compiling @angular/core : es2015 as esm2015   An unhandled exception
&gt; occurred: NGCC failed.

 

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Angular 9 - NGCC fails with an unhandled exception