When I try to save document with plot, SMath crashes and I get

$ mono SMathStudio_Desktop.exe

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object
at SMath.Control.PlotRegion.ToXml (System.Xml.XmlTextWriter writer, SMath.Control.FileParsingContext parsingContext) [0x00000] in :0
[ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object
at SMath.Control.PlotRegion.ToXml (System.Xml.XmlTextWriter writer, SMath.Control.FileParsingContext parsingContext) [0x00000] in :0

OS: Ubuntu 12.04 64 bit
Mono:
Mono JIT compiler version 2.10.8.1 (Debian 2.10.8.1-1ubuntu2.2)
Copyright (C) 2002-2011 Novell, Inc, Xamarin, Inc and Contributors. www.mono-project.com
TLS: __thread
SIGSEGV: altstack
Notifications: epoll
Architecture: amd64
Disabled: none
Misc: softdebug
LLVM: supported, not enabled.
GC: Included Boehm (with typed GC and Parallel Mark)

Did you received some help from forum?
I have the same situation here (same OS, version, bits, mono) and exactally the same error.
I just opened the beam example and tried to save it with another name.
Looking for the diff between the files, my is broken in the middle, have different xml patterns, like missing slashes, but still valid.
The saved file is pretty similar to original example file until line 1017, where the new one is broken.

flcs@flcs-desktop:~/SMath$ mono SMathStudio_Desktop.exe

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object
at SMath.Control.PlotRegion.ToXml (System.Xml.XmlTextWriter writer, SMath.Control.FileParsingContext parsingContext) [0x00000] in :0
[ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object
at SMath.Control.PlotRegion.ToXml (System.Xml.XmlTextWriter writer, SMath.Control.FileParsingContext parsingContext) [0x00000] in :0

I'd like to read or debug the source code try finding out where it's bugged.
It seems like persistence of objects to xml is the problem.
Some new structure created at PlotRegion objects design (in source code).
Maybe an inconsistent pattern or relation between objects, I suppose.

But your post was 3 months ago with no answer.