Start to fix null safety migration errors

lib/ui/widgets/settings/multi_choices_settings_tile.dart

@@ -11,15 +11,15 @@ class MultiChoicesSettingsTile<T> extends SettingsTile {
   final List<MultiChoiceEntry<T>> choices;
   final T currentValue;
   final Function(T) onChanged;
-  final Widget leading;
-  final Widget trailing;
+  final Widget? leading;
+  final Widget? trailing;
 
   MultiChoicesSettingsTile({
-    Key key,
-    @required this.title,
-    @required this.choices,
-    @required this.currentValue,
-    @required this.onChanged,
+    Key? key,
+    required this.title,
+    required this.choices,
+    required this.currentValue,
+    required this.onChanged,
     this.leading,
     this.trailing,
   })  : assert(title != null),

lib/ui/widgets/settings/text_settings_edit_tile.dart

@@ -18,15 +18,15 @@ class TextEditSettingsTile extends SettingsTile {
   final bool Function(String) checkInput;
   final bool allowEmptyValues;
   final int maxLines;
-  final int maxLength;
+  final int? maxLength;
 
   bool get readOnly => onChanged == null;
 
   TextEditSettingsTile({
-    Key key,
-    @required this.title,
-    @required this.currValue,
-    @required this.onChanged,
+    Key? key,
+    required this.title,
+    required this.currValue,
+    required this.onChanged,
     this.checkInput = _defaultCheck,
     this.allowEmptyValues = false,
     this.maxLength,
@@ -55,7 +55,7 @@ class TextEditSettingsTile extends SettingsTile {
         initialValue: currValue,
         label: title,
         checkInput: checkInput,
-        errorMessage: tr("Invalid value!"),
+        errorMessage: tr("Invalid value!")!,
         canBeEmpty: allowEmptyValues,
         maxLines: maxLines,
         maxLength: maxLength,