Fix a bug when using multiple rclpy.init context managers. (#1314)

That is, if you call rclpy.init as a context manager within
the same process, and using the default context, it currently
fails.  That is because you cannot reinitialize (call init) on
a Context object once it has been shutdown.  Instead, force
try_shutdown to replace the global context object with a new
one on rclpy.init shutdown by passing a None.

Signed-off-by: Chris Lalancette <[email protected]>

rclpy/rclpy/__init__.py

@@ -100,7 +100,12 @@ def __exit__(
         exc_val: Optional[BaseException],
         exc_tb: Optional[TracebackType],
     ) -> None:
-        try_shutdown(context=self.context, uninstall_handlers=self.installed_signal_handlers)
+        # The Context class can only be initialized once.  Thus when using the default context,
+        # we have to destroy it every time we are done using it and create a new one.
+        # utilities.try_shutdown will only replace the default context if we pass 'None', so make
+        # sure to do that for the default context.
+        shutdown_context = None if self.context is get_default_context() else self.context
+        try_shutdown(context=shutdown_context, uninstall_handlers=self.installed_signal_handlers)
 
 
 def init(