See the end of this message for details on invoking \njust-in-time (JIT) debugging instead of this dialog box.\n\n************** Exception Text **************\nSystem.NullReferenceException: Object reference not set to an instance of an object
  at Microsoft.VisualBasic.Interaction.GetSetting (System.String AppName, System.String Section, System.String Key, System.String Default) [0x00041] in <4dcb7d1ddab44fa4bafe41122e645da4>:0 
  at WindowsISODownloader.Form1.Form1_Load (System.Object sender, System.EventArgs e) [0x010f1] in <c617174d66854e919bf38dd8cb4bd484>:0 
  at System.Windows.Forms.Form.OnLoad (System.EventArgs e) [0x000d1] in <4b7d326f792d4378bd436300c6961411>:0 
  at System.Windows.Forms.Form.OnCreateControl () [0x00031] in <4b7d326f792d4378bd436300c6961411>:0 
  at System.Windows.Forms.Control.CreateControl (System.Boolean fIgnoreVisible) [0x000ed] in <4b7d326f792d4378bd436300c6961411>:0 
  at System.Windows.Forms.Control.CreateControl () [0x00008] in <4b7d326f792d4378bd436300c6961411>:0 
  at System.Windows.Forms.Control.WmShowWindow (System.Windows.Forms.Message& m) [0x00051] in <4b7d326f792d4378bd436300c6961411>:0 
  at System.Windows.Forms.Control.WndProc (System.Windows.Forms.Message& m) [0x0071b] in <4b7d326f792d4378bd436300c6961411>:0 
  at System.Windows.Forms.ScrollableControl.WndProc (System.Windows.Forms.Message& m) [0x00043] in <4b7d326f792d4378bd436300c6961411>:0 
  at System.Windows.Forms.ContainerControl.WndProc (System.Windows.Forms.Message& m) [0x0001a] in <4b7d326f792d4378bd436300c6961411>:0 
  at System.Windows.Forms.Form.WmShowWindow (System.Windows.Forms.Message& m) [0x00013] in <4b7d326f792d4378bd436300c6961411>:0 
  at System.Windows.Forms.Form.WndProc (System.Windows.Forms.Message& m) [0x00290] in <4b7d326f792d4378bd436300c6961411>:0 
  at System.Windows.Forms.Control+ControlNativeWindow.OnMessage (System.Windows.Forms.Message& m) [0x00001] in <4b7d326f792d4378bd436300c6961411>:0 
  at System.Windows.Forms.Control+ControlNativeWindow.WndProc (System.Windows.Forms.Message& m) [0x000b3] in <4b7d326f792d4378bd436300c6961411>:0 
  at System.Windows.Forms.NativeWindow.Callback (System.Windows.Forms.Message& m) [0x00025] in <4b7d326f792d4378bd436300c6961411>:0 

\n*

I don't think that you can run software on Ubuntu that's based on the .NET Framework 4. Others have come across very similar error messages, like here:

windows - Use Mono to run a software that needs Microsoft .NET Framework 4 - Ask Ubuntu

However, I've not been able to find any forum threads with the solution to this issue...