Somewhat fix errors when adding an applicant

Gui/Employees/ApplicantForm.qml

@@ -95,24 +95,33 @@ ColumnLayout {
             id: street
 
             label: qsTr("Straße")
+            mandatory: true
 
             TextField {
                 implicitWidth: fieldM
                 placeholderText: qsTr("Musterstraße")
+
+                validator: NotEmptyValidator {
+                }
             }
         }
         Field {
             id: houseno
+            mandatory: true
 
             label: qsTr("Hausnummer")
 
             TextField {
                 implicitWidth: fieldS
                 placeholderText: qsTr("1a")
+
+                validator: NotEmptyValidator {
+                }
             }
         }
         Field {
             label: qsTr("PLZ")
+            mandatory: true
 
             ComboBox {
                 id: postcode
@@ -125,6 +134,8 @@ ColumnLayout {
 
                 onActivated: currentValue
                 onCurrentIndexChanged: city.currentIndex = postcode.currentIndex
+
+                validator: NotEmptyValidator {}
             }
         }
         Field {