Workaround:
public Form1()
{
    InitializeComponent(););

    this.richTextBox1.Text = "thess is is a tesssset";

    this.radSpellChecker1.AutoSpellCheckControl = this.richTextBox1;
    this.radSpellChecker1.SpellingFormShowing += radSpellChecker1_SpellingFormShowing;
}

private void radSpellChecker1_SpellingFormShowing(object sender, SpellingFormShowingEventArgs e)
{
    e.SpellingForm.FormClosed -= SpellingForm_FormClosed;
    e.SpellingForm.FormClosed += SpellingForm_FormClosed;
}

private void SpellingForm_FormClosed(object sender, FormClosedEventArgs e)
{
    this.radSpellChecker1.AutoSpellCheckControl = null;
    this.radSpellChecker1.AutoSpellCheckControl = this.richTextBox1;
}