Simulation support: make sure that we have a data store before return…

…ing our number of runs (#1630).

This is needed, for example, when a simulation was running and then we
reloaded a SED-ML file as a result of its corresponding CellML file
having been modified.

src/plugins/support/SimulationSupport/src/simulation.cpp

@@ -676,7 +676,7 @@ int SimulationResults::runsCount() const
 {
     // Return the number of runs held by our data store
 
-    return mDataStore->runsCount();
+    return mDataStore?mDataStore->runsCount():0;
 }
 
 //==============================================================================