This probably happens due to proguard removing your Fragments.
To reproduce, build the obfuscated APK, enable "Don't Keep Activities" in developer options, open the Activity wich contains the Fragment that crashes. Minimize on home button and restore app from recents.
To merge proguard configurations with default one and yours in ADT you should specify in project.properties
proguard.config=${sdk.dir}/tools/proguard/proguard-android.txt:proguard-project.txt
If you're using Gradle build system
buildTypes {
debug {
runProguard false
}
release {
runProguard true
proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-project.txt'
}
}
And proguard-project.txt should contain at least these rules
-keep public class * extends android.preference.PreferenceFragment
And if you're using support Fragments
-keep public class * extends android.support.v4.app.Fragment
-keep public class * extends android.support.v4.app.FragmentActivity
Don't forget that there are already some rules contained in ${sdk.dir}/tools/proguard/proguard-android.txt so add only missing based on your needs.