Custom performance monitor in BTRoot produces erros when node is removed from tree

[Earewien]

Errors are displayed in error output when a node is queue free (so removed from tree) :

E 0:00:10:0020   get_custom_monitor: Error calling from custom monitor 'BTRoot/BTRoot-33319552279' to callable: '_compute_last_exec_time': Instance is null
  <C++ Error>    Condition "error" is true. Returning: return_value
  <C++ Source>   main/performance.cpp:240 @ get_custom_monitor()
E 0:00:10:0024   tick: Value of custom monitor 'BTRoot/BTRoot-33319552279' is not a number
  <C++ Source>   core/debugger/remote_debugger.cpp:79 @ tick()

In BTRoot node, a custom performance monitor is created if not running insde the Godot Editor. But, this monitor is never removed, so when the node become invalid (free, remove from scene), errors happen.

A safer way would be to create this custom monitor on tree entered, and remove it on tree exited.