OO-1912: don't open the popup if it already exists

src/main/java/org/olat/course/nodes/gta/ui/GTACoachedParticipantGradingController.java

@@ -127,6 +127,8 @@ public class GTACoachedParticipantGradingController extends BasicController {
 	}
 
 	private void doOpenAssessmentForm(UserRequest ureq) {
+		if(assessmentForm != null) return;//already open
+		
 		ICourse course = CourseFactory.loadCourse(courseOres);
 		UserCourseEnvironment uce = AssessmentHelper.createAndInitUserCourseEnvironment(assessedIdentity, course);
 		AssessedIdentityWrapper assessedIdentityWrapper = AssessmentHelper.wrapIdentity(uce, null, gtaNode);