Fixed bugs in serializing State from Python

wrappers/python/src/swig_doxygen/swig_lib/python/extend.i

@@ -338,6 +338,7 @@ Parameters:
       forces = []
       kineticEnergy = 0.0
       potentialEnergy = 0.0
+      params = {}
       types = 0
       try:
         positions = pythonState.getPositions().value_in_unit(unit.nanometers)

wrappers/python/src/swig_doxygen/swig_lib/python/pythoncode.i

@@ -192,6 +192,8 @@ class State(_object):
            See the following for details:
            https://simtk.org/home/python_units
         """
+        if self._eK0 is None:
+            raise TypeError('Energy was not requested in getState() call, so it is not available.')
         return self._eK0 * unit.kilojoule_per_mole
 
     def getPotentialEnergy(self):
@@ -201,11 +203,15 @@ class State(_object):
            See the following for details:
            https://simtk.org/home/python_units
         """
+        if self._eP0 is None:
+            raise TypeError('Energy was not requested in getState() call, so it is not available.')
         return self._eP0 * unit.kilojoule_per_mole
 
     def getParameters(self):
         """Get a map containing the values of all parameters.
         """
+        if self._paramMap is None:
+            raise TypeError('Parameters were not requested in getState() call, so are not available.')
         return self._paramMap