Crash in SVG export of a GroupWorkflow's workflow

[PathogenDavid]

Found while writing the issue for #1792

I'm not 100% sure what conditions cause this, but I wanted to get this written down before I forgot.

1. Follow all of the steps outlined in Replacing a node with a group workflow when it receives a visualizer mapping generates a bogus source node #1792
2. Go to the GroupWorkflow tab
3. Go to File > Export > Image...

Bonsai will crash with a NullReferenceException with the following stack trace:

>	SVG.dll!SvgNet.SvgFactory.LoadFromXML(System.Xml.XmlDocument doc, System.Xml.XmlElement el) Line 79	C#
 	Bonsai.Editor.dll!Bonsai.Editor.GraphView.SvgRendererFactory.TryGetIconRenderer(Bonsai.Editor.GraphModel.ElementIcon icon, out Bonsai.Editor.GraphView.SvgRenderer renderer) Line 675	C#
 	Bonsai.Editor.dll!Bonsai.Editor.GraphView.SvgRendererFactory.GetIconRenderer(Bonsai.Editor.GraphModel.GraphNode node) Line 627	C#
 	Bonsai.Editor.dll!Bonsai.Editor.GraphView.GraphViewControl.DrawNode(SvgNet.Interfaces.IGraphics graphics, Bonsai.Editor.GraphView.GraphViewControl.LayoutNode layout, System.Drawing.Size offset, System.Drawing.SolidBrush selection, System.Drawing.SolidBrush fill, System.Drawing.Pen stroke, System.Drawing.Color currentColor, System.Drawing.Color? cursor, bool hot, System.Drawing.Font vectorFont) Line 1099	C#
 	Bonsai.Editor.dll!Bonsai.Editor.GraphView.GraphViewControl.canvas_Paint(object sender, System.Windows.Forms.PaintEventArgs e) Line 1247	C#
 	System.Windows.Forms.dll!System.Windows.Forms.Control.OnPaint(System.Windows.Forms.PaintEventArgs e)	Unknown
 	<snipped>

Within SvgNet, the crash happens here: SvgFactory.LoadFromXml specifically because _elementNameDictionary["WorkflowBuilder"] is null (or rather it's missing from the XML document.)

Something seemingly nonsensical is happening. Looking up one level at TryGetIconRenderer, the icon being "opened" is ReplaceNodeWithGroupWorkflowIssue-GroupWorkflow.svg (which is the file I was trying to save.) At the time of crashing this file contains what appears to be my GroupWorkflow saved as a Bonsai workflow:

<?xml version="1.0" encoding="utf-8"?>
<WorkflowBuilder Version="2.8.1+b39a77efc5e3aa0024017fe253bb3b1d4417bbcc"
                 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                 xmlns="https://bonsai-rx.org/2018/workflow">
  <Workflow>
    <Nodes>
      <Expression xsi:type="WorkflowInput">
        <Name>Source1</Name>
      </Expression>
      <Expression xsi:type="WorkflowOutput" />
    </Nodes>
    <Edges>
      <Edge From="0" To="1" Label="Source1" />
    </Edges>
  </Workflow>
</WorkflowBuilder>

No idea what's happening here, definitely need to look into it more.