I just encountered this kind of error when using the MonoGame Content Pipeline and it's my first time to use it to convert png to xnb files (I use a stand alone app for converting files to xnb in the past). I've done building spritefonts to xnb files and stuff like that in my previous MonoGame projects and I thought it would be similar but it turned out to be not the case.
So this is the error I have:
error: Importer 'TextureImporter' had unexpected failure!
System.NullReferenceException: Object reference not set to an instance of an object
at Microsoft.Xna.Framework.Content.Pipeline.TextureImporter.Import (System.String filename, Microsoft.Xna.Framework.Content.Pipeline.ContentImporterContext context) [0x00000] in <filename unknown>:0
at Microsoft.Xna.Framework.Content.Pipeline.ContentImporter`1[Microsoft.Xna.Framework.Content.Pipeline.Graphics.TextureContent].Microsoft.Xna.Framework.Content.Pipeline.IContentImporter.Import (System.String filename, Microsoft.Xna.Framework.Content.Pipeline.ContentImporterContext context) [0x00000] in <filename unknown>:0
at MonoGame.Framework.Content.Pipeline.Builder.PipelineManager.ProcessContent (MonoGame.Framework.Content.Pipeline.Builder.PipelineBuildEvent pipelineEvent) [0x00000] in <filename unknown>:0
Is there a way to resolve this? Is there a tutorial of some kind I can follow to use the TextureImporter and the things I haven't yet understood on MonoGame's Content Pipeline with in-depth explanation about it?