警报对话框关闭而不按按钮，活动进入主页

I'm building an android app with a Login activity. If email is not valid, I want to show an alertbox with a "Email not valid" message and want to stay at same activity. The problem is that the alertbox is shown only for a brief time and then I go back immediately to the parent activity, without pressing any button in the alertbox. my code is:I'm building an android app with a Login activi