App is crashig due to Settings menu

App is crashing whenever I tap on Settings menu. Behavior is same on different Android version. But the Settings menu works if I build with following line remarked at SettingsActivity.java:

// bindPreferenceSummaryToValue(manager.findPreference(CallApplication.PREFERENCE_SOURCE));

Crash log:

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.github.axet.callrecorder, PID: 10948 java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference at com.github.axet.androidlibrary.activities.AppCompatSettingsThemeActivity$1.onPreferenceChange(AppCompatSettingsThemeActivity.java:27) at com.github.axet.androidlibrary.activities.AppCompatSettingsThemeActivity.bindPreferenceSummaryToValue(AppCompatSettingsThemeActivity.java:50) at com.github.axet.callrecorder.activities.SettingsActivity$GeneralPreferenceFragment.initPrefs(SettingsActivity.java:180) at com.github.axet.callrecorder.activities.SettingsActivity$GeneralPreferenceFragment.onCreatePreferences(SettingsActivity.java:199) at android.support.v7.preference.PreferenceFragmentCompat.onCreate(PreferenceFragmentCompat.java:223) at android.support.v4.app.Fragment.performCreate(Fragment.java:2180) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1244) at android.support.v4.app.FragmentTransition.addToFirstInLastOut(FragmentTransition.java:1085) at android.support.v4.app.FragmentTransition.calculateFragments(FragmentTransition.java:976) at android.support.v4.app.FragmentTransition.startTransitions(FragmentTransition.java:95) at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2146) at android.support.v4.app.FragmentManagerImpl.optimizeAndExecuteOps(FragmentManager.java:2103) at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2013) at android.support.v4.app.FragmentController.execPendingActions(FragmentController.java:388) at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:607) at android.support.v7.app.AppCompatActivity.onStart(AppCompatActivity.java:178) at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1435) at android.app.Activity.performStart(Activity.java:8024) at android.app.ActivityThread.handleStartActivity(ActivityThread.java:3475) at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:221) at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:201) at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:173) at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:97) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loop(Looper.java:223) at android.app.ActivityThread.main(ActivityThread.java:7656) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)