fix exception while saving file.

An error has occured while executing Python code:

Traceback (most recent call last):
  File "C:\Users\scotth/.qgis2/python/plugins\simplesvg\SimpleSvg.py", line 142, in writeToFile
    output = self.writeSVG()
  File "C:\Users\scotth/.qgis2/python/plugins\simplesvg\SimpleSvg.py", line 207, in writeSVG
    svg.extend(self.writeVectorLayer(layer, False))
  File "C:\Users\scotth/.qgis2/python/plugins\simplesvg\SimpleSvg.py", line 330, in writeVectorLayer
    if len(symbolFeatureMap[symbol])==0:
KeyError: None

Python version:
2.7.5 (default, May 15 2013, 22:44:16) [MSC v.1500 64 bit (AMD64)]