[TASK] Make category selection mandatory in ListByCategory plugin

Configuration/FlexForms/ListByCategory.xml

@@ -31,6 +31,7 @@
 								<type>select</type>
 								<renderType>selectMultipleSideBySide</renderType>
 								<size>5</size>
+								<minitems>1</minitems>
 								<maxitems>99</maxitems>
 								<foreign_table>pages</foreign_table>
 								<foreign_table_where>AND pages.sys_language_uid IN (-1, 0) AND pages.doktype = 117 ORDER BY pages.title</foreign_table_where>