fixed bug on entering themes without teachers

src/main/resources/templates/theme.html

@@ -51,7 +51,7 @@
 						</div>
 					</div>
 				</th:block>
-				<form th:action="@{/theme/update?slotId={id}(id=${slotId})}" method="post" th:object="${lesson}">
+				<form th:action="@{/theme/update?slotId={id}(id=${slotId})}" method="post" th:object="${lesson}" th:if="${lesson}">
 					<input type="hidden" th:field="*{pkLesson}" />
 					<div class="row g-2 blockframe">
 						<div class="col-sm-3">Anmerkungen zur Stunde</div>