Throw an exception if getCommonMesh cannot find a common mesh

src/scheme/DiscreteFunctionUtils.hpp

@@ -4,6 +4,7 @@
 #include <scheme/DiscreteFunctionType.hpp>
 #include <scheme/IDiscreteFunction.hpp>
 #include <scheme/IDiscreteFunctionDescriptor.hpp>
+#include <utils/Exceptions.hpp>
 
 #include <vector>
 
@@ -30,7 +31,7 @@ getCommonMesh(const std::vector<std::shared_ptr<const IDiscreteFunction>>& discr
       i_mesh = discrete_function->mesh();
     } else {
       if (i_mesh != discrete_function->mesh()) {
-        return {};
+        throw NormalError("discrete functions are not defined on the same mesh");
       }
     }
   }