Fix for nullptr dereferencing that caused test failures (#3527)

* Prevent child integrators from accessing deleted context

* Prevent reading property names from nullptr platform

* Don't check properties in a loop, but only when there's a Platform

* Clarified condition in "if" block
Co-authored-by: Peter Eastman <peter.eastman@gmail.com>

openmmapi/src/CompoundIntegrator.cpp

@@ -107,8 +107,10 @@ void CompoundIntegrator::initialize(ContextImpl& context) {
 }
 
 void CompoundIntegrator::cleanup() {
-    for (int i = 0; i < integrators.size(); i++)
+    for (int i = 0; i < integrators.size(); i++) {
         integrators[i]->cleanup();
+        integrators[i]->context = nullptr;
+    }
 }
 
 vector<string> CompoundIntegrator::getKernelNames() {

openmmapi/src/ContextImpl.cpp

@@ -93,8 +93,9 @@ ContextImpl::ContextImpl(Context& owner, const System& system, Integrator& integ
     
     // Validate the list of properties.
 
-    const vector<string>& platformProperties = platform->getPropertyNames();
     map<string, string> validatedProperties;
+    if (platform != NULL) {
+        const vector<string>& platformProperties = platform->getPropertyNames();
         for (auto& prop : properties) {
             string property = prop.first;
             if (platform->deprecatedPropertyReplacements.find(property) != platform->deprecatedPropertyReplacements.end())
@@ -109,6 +110,10 @@ ContextImpl::ContextImpl(Context& owner, const System& system, Integrator& integ
                 throw OpenMMException("Illegal property name: "+prop.first);
             validatedProperties[property] = prop.second;
         }
+    }
+    else {
+        // There can't be any platform-specific properties if there's no platform
+    }
 
     // Find the list of kernels required.