NullReference when importing custom midis on UnitySynth

So, I downloaded the unity project from unity answers. here is the link:

UnitySynth link to download page in unity answers

I have finished my prototype, but I'm getting errors when I tried to load a custom midi I downloaded from the web.

I got these error codes:

 Error Loading Midi:
 Midi Failed to Load!
 UnityEngine.Debug:Log(Object)
 CSharpSynth.Sequencer.MidiSequencer:LoadMidi(String, Boolean) (at Assets/Plugins/CSharpSynth/Sequencer/MidiSequencer.cs:153)
 UnitySynthTest:Awake() (at Assets/Scripts/UnitySynthTest.cs:42)
 
 NullReferenceException: Object reference not set to an instance of an object
 CSharpSynth.Sequencer.MidiSequencer.Play () (at Assets/Plugins/CSharpSynth/Sequencer/MidiSequencer.cs:167)
 UnitySynthTest.OnGUI () (at Assets/Scripts/UnitySynthTest.cs:161)

I would love to know an easy way to convert midis to mid.txt as the examples used in the demo.

EDIT:

The first error code, the one about not loading midi, comes from here:

 public bool LoadMidi(string file, bool UnloadUnusedInstruments)
 {
     if (playing == true)
         return false;
     MidiFile mf = null;
     try
     {
         mf = new MidiFile(file);
     }
     catch (Exception ex)
     {
         //UnitySynth
 
 //THIS IS THE ERROR LINE **************************
             Debug.Log("Error Loading Midi:\n" + ex.Message);
 
 return false;
     }
     return LoadMidi(mf, UnloadUnusedInstruments);
 }

The second one, the one about null reference exception comes from:

     public void Play()
     {
         if (playing == true)
             return;
         //Clear the current programs for the channels.
         Array.Clear(currentPrograms, 0, currentPrograms.Length);
         //Clear vol, pan, and tune
         ResetControllers();
         //set bpm
 
 // THIS IS THE ERROR LINE *****************************
      _MidiFile.BeatsPerMinute = 120;
 
 
         //Let the synth know that the sequencer is ready.
         eventIndex = 0;
         playing = true;
     }