Block decomposition: fix crash when a variable appears in an equation but has zero symbolic derivative

src/DynamicModel.cc

@@ -4441,7 +4441,11 @@ DynamicModel::computeChainRuleJacobian()
           switch (derivType)
             {
             case BlockDerivativeType::standard:
-              d = derivatives[1][{ eq_orig, deriv_id }];
+              if (auto it = derivatives[1].find({ eq_orig, deriv_id });
+                  it != derivatives[1].end())
+                d = it->second;
+              else
+                d = Zero;
               break;
             case BlockDerivativeType::chainRule:
               d = equations[eq_orig]->getChainRuleDerivative(deriv_id, recursive_vars);

src/ModelTree.hh

@@ -106,7 +106,7 @@ protected:
      For each derivation order, stores a map whose key is a vector of integer: the
      first integer is the equation index, the remaining ones are the derivation
      IDs of variables (in non-decreasing order, to avoid storing symmetric
-     elements several times) */
+     elements several times). Only non-zero derivatives are stored. */
   vector<map<vector<int>, expr_t>> derivatives;
 
   //! Number of non-zero derivatives