Android Studio build项目报错 而且R文件找不到

[柒加伊]

D:\workspace\MyApplication>gradlew build
To honour the JVM settings for this build a new JVM will be forked. Please consider using the daemon: https://docs.gradle.org/2.14.1/userguide/gradle_daemon.html.
:app:preBuild UP-TO-DATE
:app:preDebugBuild UP-TO-DATE
:app:checkDebugManifest
:app:preReleaseBuild UP-TO-DATE
:app:prepareComAndroidSupportAnimatedVectorDrawable2421Library UP-TO-DATE
:app:prepareComAndroidSupportAppcompatV72421Library UP-TO-DATE
:app:prepareComAndroidSupportSupportCompat2421Library UP-TO-DATE
:app:prepareComAndroidSupportSupportCoreUi2421Library UP-TO-DATE
:app:prepareComAndroidSupportSupportCoreUtils2421Library UP-TO-DATE
:app:prepareComAndroidSupportSupportFragment2421Library UP-TO-DATE
:app:prepareComAndroidSupportSupportMediaCompat2421Library UP-TO-DATE
:app:prepareComAndroidSupportSupportV42421Library UP-TO-DATE
:app:prepareComAndroidSupportSupportVectorDrawable2421Library UP-TO-DATE
:app:prepareComJakewhartonButterknife840Library UP-TO-DATE
:app:prepareIoReactivexRxandroid121Library

FAILURE: Build failed with an exception.

* What went wrong:
java.lang.NullPointerException (no error message)

[碼农李泽斌]

clean一下工程，然后ReBuild