RUBY-24836 fixup

Author: viuginick1

lib/ruby-debug-ide/xml_printer.rb

@@ -161,6 +161,11 @@ def do_print_hash(hash)
           else
             name = exec_with_allocation_control(k, :to_s, OverflowMessageType::EXCEPTION_MESSAGE)
           end
+
+          if k.nil?
+            name = 'nil'
+          end
+
           print_variable(name, v, 'instance')
         }
       end