private void CalculateButton_Click(object sender, EventArgs e) { if (HeightTextBox == null || HeightTextBox.Text == "") { BMIRichTextBox.Clear(); HeightTextBox.AppendText("Text cannot be null"); } if (WeightTextBox == null || WeightTextBox.Text == "") { BMIRichTextBox.Clear(); WeightTextBox.AppendText("Text cannot be null"); } else { BMIRichTextBox.Clear(); Calculate(); } }