refactor Dropdown

- load dropdown only if dropdown before was chosen
- validation if field in between is empty
- ensure no null value gets into route

system/phrasesupdate.php

@@ -51768,6 +51768,46 @@ I have been informed that I am under no obligation to consent to the transmissio
 			)
 		)
 	),
+	array(
+		'app' => 'core',
+		'category' => 'LvPlan',
+		'phrase' => 'error_SemMissing',
+		'insertvon' => 'system',
+		'phrases' => array(
+			array(
+				'sprache' => 'German',
+				'text' => 'Semester wählen',
+				'description' => '',
+				'insertvon' => 'system'
+			),
+			array(
+				'sprache' => 'English',
+				'text' => 'Select a semester',
+				'description' => '',
+				'insertvon' => 'system'
+			)
+		)
+	),
+	array(
+		'app' => 'core',
+		'category' => 'LvPlan',
+		'phrase' => 'error_VerbandMissing',
+		'insertvon' => 'system',
+		'phrases' => array(
+			array(
+				'sprache' => 'German',
+				'text' => 'Verband wählen',
+				'description' => '',
+				'insertvon' => 'system'
+			),
+			array(
+				'sprache' => 'English',
+				'text' => 'Select a verband',
+				'description' => '',
+				'insertvon' => 'system'
+			)
+		)
+	),
 	// ### 		LvPlanStgOrg 		END 		###
 );