Help No Main Class Specified error after upgrade of Android Studio

[Orawolf]

Hello,

So, I have this app that was running like a charm in previous releases of Android Studio. But after I upgraded to Studio's latest release android-studio-bundle-141.2135290-windows, The APP has stopped working.

Here are the issues now.

1. When I enter Run Under "Edit Configuration" Window I get "No Main Class Specified". My App never had a main class. So why it was working fine now?

2. When I still click the Run, It says "Invalid Configuration", but I attempt to ignore that and continue running, then it gives following set of errors. I have downloaded all SDK Manager Packages and JDK is properly installed. Gradle is also installed. Not sure why below errors are coming. Any Idea?

Information:Using javac 1.8.0_45 to compile java sources
Information:java: Errors occurred while compiling module 'QuizApp'
Information:8/18/2015 9:19 PM - Compilation completed with 100 errors and 0 warnings in 2s 689ms
C:\mobile\AndroidQuizApp\QuizApp\src\com\androidgiri\quizapp\SetLevelActivity.java
Error3, 19) java: package android.app does not exist
Error4, 18) java: package android.os does not exist
Error5, 20) java: package android.view does not exist
Error6, 25) java: package android.view.View does not exist
Error7, 22) java: package android.widget does not exist
Error8, 22) java: package android.widget does not exist
Error11, 39) java: cannot find symbol
symbol: class Activity
Error11, 59) java: cannot find symbol
symbol: class OnClickListener
Error13, 9) java: cannot find symbol
symbol: class RadioButton
location: class com.androidgiri.quizapp.SetLevelActivity
Error14, 9) java: cannot find symbol
symbol: class RadioButton
location: class com.androidgiri.quizapp.SetLevelActivity
Error15, 9) java: cannot find symbol
symbol: class RadioButton
location: class com.androidgiri.quizapp.SetLevelActivity
Error18, 33) java: cannot find symbol
symbol: class Bundle
location: class com.androidgiri.quizapp.SetLevelActivity
Error38, 29) java: cannot find symbol
symbol: class View
location: class com.androidgiri.quizapp.SetLevelActivity
Error17, 9) java: method does not override or implement a method from a supertype
Error19, 17) java: cannot find symbol
symbol: variable super
location: class com.androidgiri.quizapp.SetLevelActivity
Error20, 33) java: package R does not exist
Error22, 17) java: cannot find symbol
symbol: class Button
location: class com.androidgiri.quizapp.SetLevelActivity
Error22, 40) java: cannot find symbol
symbol: class Button
location: class com.androidgiri.quizapp.SetLevelActivity