Issue title: Visual Studio 2019 Error
 Issue body: ### Installed product versions
- Visual Studio: Visual Studio 2019 Enterprise
- This extension: 2.8.396

### Description
Error when saving js file that should be minfied:
System.NullReferenceException: Object reference not set to an instance of an object.
   at BundlerMinifier.BundleFileProcessor.SourceFileChanged(String bundleFile, String sourceFile)
   at BundlerMinifierVsix.BundleService.<>c__DisplayClass11_0.<SourceFileChanged>b__0(Object o) in C:\projects\bundlerminifier\src\BundlerMinifierVSIX\BundleService.cs:line 169


### Steps to recreate
1.  Create js file
2. Add file to bundleconfig.json
3. Save file

### Current behavior
No min file is created and no minification is done

### Expected behavior
min file should be created and the js file should be minified.
 Comments: 
 Comment 0: Never mind, my stupid error... Missing a comma.
 Comment 1: Same here, got the error when trying to minify the bootstrap.js 5.0. 
When minify is disabled, it works fine with just bundling. Any solution or fixes?
{'source': 'GithubIssues', 'repo_name_with_owner': 'madskristensen/BundlerMinifier', 'issue_id': 430460730, 'repo_stars': 631.0}