use getTargetClass in function checkTargetAttributeExistence
use getTargetClass in function checkTargetAttributeExistence

This interesting change, to be able to change getTargetClass when necessary when extending this class.
mikehaertl/yii2,xiaoliwang/yii2,bizley/yii2,LAV45/yii2,rhertogh/yii2,PowerGamer1/yii2,arogachev/yii2,yiisoft/yii2,mikk150/yii2,brandonkelly/yii2,arogachev/yii2,mikk150/yii2,bizley/yii2,brandonkelly/yii2,mikehaertl/yii2,yiijan/yii2,xiaoliwang/yii2,PowerGamer1/yii2,LAV45/yii2,bizley/yii2,rhertogh/yii2,PowerGamer1/yii2,mikk150/yii2,lubosdz/yii2,mikehaertl/yii2,yiijan/yii2,yiisoft/yii2,yiisoft/yii2,lubosdz/yii2,brandonkelly/yii2,yiijan/yii2,arogachev/yii2,xiaoliwang/yii2,arogachev/yii2,brandonkelly/yii2,lubosdz/yii2,rhertogh/yii2,mikk150/yii2,mikehaertl/yii2,yiijan/yii2,lubosdz/yii2,rhertogh/yii2,LAV45/yii2,LAV45/yii2,yiisoft/yii2,mikk150/yii2,bizley/yii2,PowerGamer1/yii2,LAV45/yii2,xiaoliwang/yii2,yiijan/yii2