Workaround for new blueprints. Force select with null value to empty string

themes/grav/app/forms/state.js

@@ -90,6 +90,7 @@ export default class FormState {
             field = $(field);
             let name = field.prop('name');
             let type = field.prop('type');
+            let tag = field.prop('tagName').toLowerCase();
             let value;
 
             switch (type) {
@@ -101,6 +102,10 @@ export default class FormState {
                     value = field.val();
             }
 
+            if (tag === 'select' && value === null) {
+                value = '';
+            }
+
             if (name && !~this.options.ignore.indexOf(name)) {
                 values[name] = value;
             }