Visual Studio 2015 only allows removal in Typescript files

Question

For some reason, I can no longer add TypeScript files to VS2015. I can delete things and undo these deletions, but actually I can’t enter anything.

It started a bit after I installed the Markdown Mode extension, but disabling it doesn't fix anything.

I can edit the transferred JS files, which is strange. These seem to be only .ts files.

Edit: I believe this has something to do with this:

<record>596</record>
<time>2016/05/31 17:52:41.977</time>
<type>Error</type>
<source>Editor or Editor Extension</source>
<description>System.NullReferenceException: Object reference not set to an instance of an object.&#x000D;&#x000A;   at Microsoft.VisualStudio.Web.Editor.Package.Lint.WebLinter.LinterUtilities.IsFileSupported(String fileName)&#x000D;&#x000A;   at Microsoft.VisualStudio.Web.Editor.Package.Lint.ErrorTagger.LintErrorTaggerProvider.CreateTagger[T](ITextBuffer textBuffer)&#x000D;&#x000A;   at Microsoft.VisualStudio.Text.Tagging.Implementation.TagAggregator`1.GatherTaggers(ITextBuffer textBuffer)</description>

Answer 1

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