Created Unassigned: iOS Adding an Axis to Model then Rendering Throws NullReferenceException [10161]

See discussion - https://oxyplot.codeplex.com/discussions/538962
See the running example code in the OxyPlot.XamarinIOS Example Browser app in the fork [https://oxyplot.codeplex.com/SourceControl/network/forks/benhysell/PlotViewModelIssue](https://oxyplot.codeplex.com/SourceControl/network/forks/benhysell/PlotViewModelIssue)

With ```PlotView``` unsealed I can now do the following:
```
public class GraphViewBad : PlotView
{

public GraphViewBad ()
{

}

public void BuildGraph()
{
Model = new PlotModel ("Bad Plot");
Model.TitleFontSize = 12;
Model.TitleFont = "Helvetica";
Model.TitleFontWeight = FontWeights.Normal;
Model.TitlePadding = 0;
Model.Padding = new OxyPlot.OxyThickness (3, 0, 0, 12);
Model.PlotAreaBackground = OxyColors.White;
Model.PlotAreaBorderThickness = 0;
Model.Background = OxyColors.Red;

//once an axis is added application fails with null ref
Model.Axes.Add (new LinearAxis (AxisPosition.Left) {
TickStyle = TickStyle.Outside,
AxislineStyle = LineStyle.Solid,
MajorStep = 1,
Minimum = 0,
Maximum = 10
});
}
}
```

Nothing fancy, a red background with a y-axis...enough to show the error.

When I run this code I get a ```System.NullReferenceException``` in ```PlotElement.cs```
```
/// <summary>
/// Gets the actual font.
/// </summary>
protected internal string ActualFont
{
get
{
return this.Font ?? this.PlotModel.DefaultFont;
}
}
```

If I change my code to the following the view displays as expected:
```
public class GraphViewGood : PlotView
{
PlotModel model;

public GraphViewGood ()
{

}

public void BuildGraph()
{
model = new PlotModel ("Good Plot");
model.TitleFontSize = 12;
model.TitleFont = "Helvetica";
model.TitleFontWeight = FontWeights.Normal;
model.TitlePadding = 0;
model.Padding = new OxyPlot.OxyThickness (3, 0, 0, 12);
model.PlotAreaBackground = OxyColors.White;
model.PlotAreaBorderThickness = 0;
model.Background = OxyColors.Red;

model.Axes.Add (new LinearAxis (AxisPosition.Left) {
TickStyle = TickStyle.Outside,
AxislineStyle = LineStyle.Solid,
MajorStep = 1,
Minimum = 0,
Maximum = 10
});
Model = model;
}
}
```