signature null values fix.

lib/views/widgets/e_signature/e_signature.dart

@@ -19,7 +19,8 @@ class ESignature extends StatefulWidget {
   final bool showShadow;
   final String title;
 
-  const ESignature({Key? key, required this.title, required this.oldSignature, this.backgroundColor, this.onSaved, this.onChange, this.showShadow=true, required this.newSignature}) : super(key: key);
+  const ESignature({Key? key, required this.title, required this.oldSignature, this.backgroundColor, this.onSaved, this.onChange, this.showShadow = true, required this.newSignature})
+      : super(key: key);
 
   @override
   State<ESignature> createState() => _ESignatureState();
@@ -89,7 +90,7 @@ class _ESignatureState extends State<ESignature> {
                         .onPress(() async {
                       if (_editable) {
                         signature = await _controller!.toPngBytes();
-                        if (widget.onChange != null) {
+                        if (widget.onChange != null && signature != null) {
                           widget.onChange!(signature!);
                         }
                       }