Mfrac.Visit() throws NullReferenceException

[Maximilian-Renner]

Here is another bug. If you use the Visit() function on any Mfrac object, it will throw a NullreferenceException. Math term represented in this example: "a/b"
The MathML code was converted by: http://www.mathmlcentral.com/Tools/ToMathML.jsp

[TestMethod]
public void ParseFormula_Mfrac()
{
    string mathMl = "<math><mfrac><mi>a</mi><mi>b</mi></mfrac></math>";
    var a = MathMLToCSharpLib.Parser.Parse(XElement.Parse(mathMl));
    StringBuilder sb = new StringBuilder();
    MathMLToCSharpLib.BuildContext bc = new MathMLToCSharpLib.BuildContext();
    a.Visit(sb, bc);
}

This is one of the easiest test case that could have been written for this library. Please do improve your testing.