update nullsafety (#749)

Author: sjoerd0301

pubspec.yaml

@@ -22,7 +22,7 @@ dependencies:
   flutter_typeahead: ^3.1.0
   intl: ^0.17.0
   rating_bar: ^0.2.0
-  signature: ^3.2.1
+  signature: ^4.0.0-nullsafety
   validators: ^2.0.1
 
 dependency_overrides: