app crashes when fetching attribute values by count or sum functions

when fetching attribute values that satisfy a given function such as count, sum etc, the app will crashes with message:
```
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Invalid property ((<NSAttributeDescription: 0x1f8a2750>), name countMember, isOptional 1, isTransient 0, entity Transaction, renamingIdentifier action, validation predicates (
), warnings (
), versionHashModifier (null)
 userInfo {
}, attributeType 100 , attributeValueClassName NSNumber, defaultValue 0) passed to setPropertiesToFetch: (entity mismatch)'.
```
The fetching snippet I used looks like below:

    NSExpressionDescription *amountDescription = [[NSExpressionDescription alloc] init];
    [amountDescription setName:@"amount"];
    [amountDescription setExpression:[NSExpression expressionForKeyPath:@"@sum.amount"]];
    [amountDescription setExpressionResultType:NSInteger32AttributeType];

    NSFetchRequest *fetchTransaction = [NSFetchRequest fetchRequestWithEntityName:@"Transaction"];
    [fetchTransaction setPredicate:[NSPredicate predicateWithFormat:@"shopId = %@", shop[@"shopId"]]];
    [fetchTransaction setPropertiesToFetch:@[@"action", amountDescription]];
    [fetchTransaction setPropertiesToGroupBy:@[@"action"]];
    [fetchTransaction setResultType:NSDictionaryResultType];

    [_managedObjectContext performBlockAndWait:^{
        NSError *error = nil;
        NSArray *transactionResults = [_managedObjectContext executeFetchRequest:fetchTransaction error:&error];
        if ([transactionResults lastObject]) {
            NSLog(@"%@", transactionResults);
        }
    }];

This fixes the bug by transfer the propertiesToFetch and propertiesToGroupBy from fetchRequest to backingFetchRequest. fetchRequest and backingFetchRequest are in different NSManagedObjectContext.