when will a NSObject's property could be null - ios

Question

I have a model translator. It reads AModel's properties, and copy the value to BModel's same property if BModel has. Now I have a crash report shows the property is null. That is so strange. The property is got from a property list and it is null.
Here is the message:
*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<XXXXXX 0x1594051a0> valueForUndefinedKey:]: this class is not key value coding-compliant for the key (null).
+ (instancetype)convertSourceObject:(id)sourceObject {
if (!sourceObject) {
return nil;
}
id destination_object = [[self alloc] init];
uint destination_properties_count = 0;
objc_property_t *destination_properties = class_copyPropertyList([self class], &destination_properties_count);
for (int i = 0; i < destination_properties_count; i++) {
objc_property_t destination_property = destination_properties[i];
uint source_attributes_count = 0, destination_attributes_count = 0;
objc_property_attribute_t *destination_attributes = property_copyAttributeList(destination_property, &destination_attributes_count);
const char *property_char_name = property_getName(destination_property);
NSString *property_name = [NSString stringWithUTF8String:property_char_name];
objc_property_t source_property = class_getProperty([sourceObject class], property_char_name);
if (source_property && ![ignorePropertyNames() containsObject:property_name]) {
objc_property_attribute_t *source_attributes = property_copyAttributeList(source_property, &source_attributes_count);
NSString *source_ivar_type = #"";
NSString *destination_ivar_type = #"";
for (int i = 0; i < source_attributes_count; i++) {
if (strcmp(source_attributes[i].name, "T") == 0) {
source_ivar_type = [[NSString stringWithUTF8String:source_attributes[i].value] stringByTrimmingCharactersInSet:[NSCharacterSet characterSetWithCharactersInString:#"#\""]];
break;
}
}
for (int i = 0; i < destination_attributes_count; i++) {
if (strcmp(destination_attributes[i].name, "T") == 0) {
destination_ivar_type = [[NSString stringWithUTF8String:destination_attributes[i].value] stringByTrimmingCharactersInSet:[NSCharacterSet characterSetWithCharactersInString:#"#\""]];
break;
}
}
if ([self isPropertySetType:source_ivar_type]) {
id source_value = [sourceObject valueForKey:property_name];
if ([source_value isKindOfClass:[NSArray class]]) {
NSArray *destination_array = [self arrayConvert:(NSArray*)source_value];
if (destination_array) {
[destination_object setValue:destination_array forKey:property_name];
}
} else if ([source_value isKindOfClass:[NSDictionary class]]) {
NSDictionary *destination_dict = [self dictionaryConvert:(NSDictionary *)source_value];
if (destination_dict) {
[destination_object setValue:destination_dict forKey:property_name];
}
}
} else {
if ([destination_ivar_type isEqualToString:source_ivar_type]) {
id source_value = [sourceObject valueForKey:property_name];
if (source_value) {
[destination_object setValue:source_value forKey:property_name];
}
} else {
id source_value = [sourceObject valueForKey:property_name];
id destination_value = [NSClassFromString(destination_ivar_type) convertSourceObject:source_value];
if (destination_value) {
[destination_object setValue:destination_value forKey:property_name];
}
}
}
free(source_attributes);
} else {
continue;
}
free(destination_attributes);
}
free(destination_properties);
return destination_object;
}

Answer 1

The error means: you try to read a property which name's "null" AModel's property , but it doesn't exit in AModel.
You should overwrite valueForUndefinedKey in your Model Class, debug the UndefinedKey.
Check your code.It seems happened at
id source_value = [sourceObject valueForKey:property_name];
NSLog debug the property_name, see what you got.