Improve error message if bad mesh variant is accessed

src/mesh/MeshVariant.hpp

 #ifndef MESH_VARIANT_HPP
 #define MESH_VARIANT_HPP
 
+#include <mesh/MeshTraits.hpp>
+#include <utils/Demangle.hpp>
+#include <utils/Exceptions.hpp>
 #include <utils/PugsMacros.hpp>
 
+#include <rang.hpp>
+
 #include <iostream>
 #include <memory>
+#include <sstream>
 #include <variant>
 
 class IConnectivity;
@@ -38,6 +44,16 @@ class MeshVariant
   PUGS_INLINE std::shared_ptr<const MeshType>
   get() const
   {
+    if (not std::holds_alternative<std::shared_ptr<const MeshType>>(this->m_p_mesh_variant)) {
+      std::ostringstream error_msg;
+      error_msg << "invalid mesh type type\n";
+      error_msg << "- required " << rang::fgB::red << demangle<MeshType>() << rang::fg::reset << '\n';
+      error_msg << "- contains " << rang::fgB::yellow
+                << std::visit([](auto&& p_mesh) -> std::string { return demangle<mesh_type_t<decltype(p_mesh)>>(); },
+                              this->m_p_mesh_variant)
+                << rang::fg::reset;
+      throw NormalError(error_msg.str());
+    }
     return std::get<std::shared_ptr<const MeshType>>(m_p_mesh_variant);
   }