When you have a coding error, it is very helpful if you post your entire code, preferably so that it can be compiled and the error can be reproduced.
Just based on what you have posted, I would check the variable declarations in the monitor and make sure that they are of the correct type. Remember that get_config_object() can only use a uvm_object type to return the handle and you will have to cast it to the appropriate type handle.
See the example in the UVM Cookbook.