Conditions #21

- Create PropertyValueGetterMixin to get a property value from conditions, variables and node values.
- Refactor visible, currentVariantId and fill properties to use getPropertyValue function.

Author: BirjuVachhani

lib/src/api/models/variables_model.dart

@@ -141,7 +141,7 @@ class VariableData
       // TODO: implement type conversion.
       throw UnimplementedError('Not Implemented');
     } else {
-      throw Exception('Unsupported type: $T');
+      return value as T?;
     }
   }
 }