Fixed clean-up of the motion library causing problems with the Python API (#6029)

* Fixed clean-up of the motion library causing problems with the Python API

* Change log

* Use ROBOT_ASSERT instead of assert

* Fixed compilation

Author: omichel

docs/reference/changelog-r2023.md

@@ -12,6 +12,7 @@ Released on ??
     - Improved default selected tab in Field Editor when nodes are selected ([#5726](https://github.com/cyberbotics/webots/pull/5726)).
     - Change the Windows version of SUMO to 1.13 to match the one used on Linux and MacOS and avoid a potiental Log4J vulnerability ([#6010](https://github.com/cyberbotics/webots/pull/6010)).
   - Bug Fixes
+    - Fixed the clean-up of the motion API which was firing warnings in Python ([#6029](https://github.com/cyberbotics/webots/pull/6029)). 
     - Fixed the behavior of the [Connector](connector.md) after a reset to return to the controller the correct status ([#5889](https://github.com/cyberbotics/webots/pull/5889))
     - Fixed redirection of stdout/stderr for Python controllers on Windows ([#5807](https://github.com/cyberbotics/webots/pull/5807)).
     - Fixed crash in Python API when a robot controller was using several cameras with different resolutions ([#5705](https://github.com/cyberbotics/webots/pull/5705)).

src/controller/c/motion.c

@@ -50,6 +50,7 @@ static const double UNDEFINED_POSITION = -9999999.9;
 static WbMotionRef head = NULL;
 static const char *HEADER = "#WEBOTS_MOTION";
 static const char *VERSION = "V1.0";
+static int cleanup_done = 0;
 
 // string to time conversion
 // acceptable input format: [[Minutes:]Seconds:]:Milliseconds
@@ -382,6 +383,7 @@ void motion_cleanup() {
     wbu_motion_delete(head);
 
   ROBOT_ASSERT(head == NULL);
+  cleanup_done = 1;
 }
 
 // Webots API functions below
@@ -451,10 +453,12 @@ WbMotionRef wbu_motion_new(const char *filename) {
 void wbu_motion_delete(WbMotionRef motion) {
   if (!motion)
     return;
-
   // dequeue self
   if (!motion_dequeue(motion)) {
-    fprintf(stderr, "Error: %s(): attempt to delete an invalid 'motion'.\n", __FUNCTION__);
+    if (cleanup_done) {            // the Python API calls first motion_cleanup and then tries to delete motion files.
+      ROBOT_ASSERT(head == NULL);  // no new motion should have been created after the cleanup.
+    } else
+      fprintf(stderr, "Error: %s(): attempt to delete an invalid 'motion'.\n", __FUNCTION__);
     return;
   }