Understanding GDB back trace with error access outside bounds of object referenced via synthetic pointer

I have 2 classes using CRTP. Class A parses a json file. Class B gets the Json data by calling get() method of Class A. Class A in turn calls set() of Class B after parsing the Json.

template <class T>
class A
  {
    private:
     nlohmann::json doc;
     using jptr = nlohmann::json::json_pointer;
     jptr addl_fld_ptr;
     public:
     parse_nf_event(std::string event)
     {
             try
             {
             doc = nlohmann::json::parse(event);
             }
             catch(const nlohmann::json::exception& e)
             {
                 std::cout<<"MVRP: Cannot create Json from string\n";
                 throw "parse_nf_event: json string invalid";
             }

         
         addl_fld_ptr="/event/path/to/msuid/"_json_pointer;
      }
      inline void get_msuid()
     {   
         //return "NULL" if field not present
         static_cast<T&>(*this).set_msuid(doc.value(addl_fld_ptr/"msuid","NULL"));
     }
};

class B:public A<B>
 {
   private:

      something *ptr;

   public:
      B(std::string str):A(str)//str is json string
      {
      }
      inline void set_msuid(std::string val)
      {
          ptr->msu_id.assign(val);
      }
      void* get_payload()
      {
        ptr = new something();
        get_msuid();
        return ptr;
      }
};

Above code is trimmed version

User calls get_payload.

Sometimes I am getting a core

(gdb) bt
#0  0x00007fc2a27cc38d in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_assign(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6

#1  0x00000000006be6ff in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::assign (__str="",
    this=<optimized out>) at /usr/local/include/c++/5.4.0/bits/basic_string.h:1095
#2  B::set_msuid (this=0x7fc298ff6b20,
    val=<error reading variable: access outside bounds of object referenced via synthetic pointer>)
    
#3  A<B>::get_msuid (this=0x7fc298ff6b20)
    
#4  B::get_payload (this=0x7fc298ff6b20)
    

This is the first time I am using CRTP, and not sure if I did it right. I am not able to understand the reason from back trace. From the Json I can see msuid="" so nlohmann parser returns empty string.