To reproduce: 
public class SwedishDictionary : WordDictionary
{
      protected override void EnsureDictionaryLoadedOverride()
     {
           using (MemoryStream ms = new MemoryStream(Properties.Resources.sv_SE))
          {
                this.Load(ms);
          }
    }
}

Workaround - call the method in the constructor: 

public class SwedishDictionary : WordDictionary
{
      public SwedishDictionary()
      {
            this.EnsureDictionaryLoaded();
      }
 
      protected override void EnsureDictionaryLoadedOverride()
     {
           using (MemoryStream ms = new MemoryStream(Properties.Resources.sv_SE))
          {
                this.Load(ms);
          }
    }
}

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.