如何让用户留下分配为Char的空白文本框

Question

我对C＃非常陌生，并试图开发GPA计算器。它有10个文本框和另一个文本框来输入课程数。当用户输入所有10个等级时它工作正常，但当用户输入少于10个等级时，它会显示错误 - ＆＃39;您的应用程序中出现了未处理的异常＆＃39;

代码的一部分 -

        TextBox[] boxes = { txbCourse1, txbCourse2, txbCourse3, txbCourse4, txbCourse5, txbCourse6, txbCourse7, txbCourse8, txbCourse9, txbCourse10 };

            //use of a loop to go over the array and find GPA for each course
        for (int i = 0; i < 10; i++)
        {
            input = Convert.ToChar(boxes[i].Text);

            if (input == 'A' || input == 'a')
                GPA = GRA;
            if (input == 'B' || input == 'b')
                GPA = GRB;
            if (input == 'C' || input == 'c')
                GPA = GRC;
            if (input == 'D' || input == 'd')
                GPA = GRD;
            if (input == 'E' || input == 'e')
                GPA = GRE;
            if (input == 'F' || input == 'f')
                GPA = GRF;

            total += GPA;
        }

        //output to the user (cumulative GPA)
            cumGPA = Math.Round(total / number,2);
            lblGPA.Text = "The Cumulative GPA: " + cumGPA;
    }