Give a proper error message if the user tries to create a context with no particles

openmmapi/src/ContextImpl.cpp

@@ -49,6 +49,8 @@ using std::string;
 
 ContextImpl::ContextImpl(Context& owner, System& system, Integrator& integrator, Platform* platform, const map<string, string>& properties) :
          owner(owner), system(system), integrator(integrator), hasInitializedForces(false), platform(platform), platformData(NULL) {
+    if (system.getNumParticles() == 0)
+        throw OpenMMException("Cannot create a Context for a System with no particles");
     vector<string> kernelNames;
     kernelNames.push_back(CalcKineticEnergyKernel::Name());
     kernelNames.push_back(CalcForcesAndEnergyKernel::Name());

platforms/reference/tests/TestReferenceMonteCarloBarostat.cpp

@@ -53,6 +53,7 @@ void testChangingBoxSize() {
     ReferencePlatform platform;
     System system;
     system.setDefaultPeriodicBoxVectors(Vec3(4, 0, 0), Vec3(0, 5, 0), Vec3(0, 0, 6));
+    system.addParticle(1.0);
     LangevinIntegrator integrator(300.0, 1.0, 0.01);
     Context context(system, integrator, platform);
     Vec3 x, y, z;