cannot import interpreter

edit

When building ROS2 on Windows using this command on Visual studio command prompt

colcon build --merge install

It shows the error:

Exception loading extension 'colcon_core.environment_variable.powershell_command': cannot import name 'Interpreter' from 'em' (c:\python37\lib\site-packages\em\__init__.py)
Traceback (most recent call last):
  File "c:\python37\lib\site-packages\colcon_core\entry_point.py", line 98, in load_entry_points
    extension_type = load_entry_point(entry_point)
  File "c:\python37\lib\site-packages\colcon_core\entry_point.py", line 140, in load_entry_point
    return entry_point.load()
  File "c:\python37\lib\site-packages\pkg_resources\__init__.py", line 2411, in load
    return self.resolve()
  File "c:\python37\lib\site-packages\pkg_resources\__init__.py", line 2417, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "c:\python37\lib\site-packages\colcon_powershell\shell\powershell.py", line 17, in <module>
    from colcon_core.shell.template import expand_template
  File "c:\python37\lib\site-packages\colcon_core\shell\template\__init__.py", line 8, in <module>
    from em import Interpreter
ImportError: cannot import name 'Interpreter' from 'em' (c:\python37\lib\site-packages\em\__init__.py)

Does anyone know how to resolve this problem?

Thanks!