Butterknife并不总是编译并产生NullPointerException

Question

您好我在这里面临一个非常奇怪的问题。我已经使用依赖注入（Butterknife）来为BindViews而不是写锅炉板代码来每次都实现视图。有时它可以解决问题，有时只会产生NullpointerException，因为它不会编译我认为的视图。

我的app模块（build.gradle）：

apply plugin: 'com.android.application'

repositories {
    maven { url "https://jitpack.io" }
}

android {
    compileSdkVersion 25
    buildToolsVersion "25.0.1"
    defaultConfig {
        applicationId "com.example.androidnative.testproject"
        minSdkVersion 15
        targetSdkVersion 25
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        vectorDrawables.useSupportLibrary = true
        jackOptions {
            enabled true
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile project(':mylibrary')
    compile 'com.android.support:appcompat-v7:25.0.1'
    compile 'com.android.support:design:25.0.1'
    compile 'com.android.support:support-v4:25.0.1'
    compile 'com.android.support:cardview-v7:25.0.1'
    compile 'com.android.support:recyclerview-v7:25.0.1'
    compile 'com.jakewharton:butterknife:8.4.0'
    annotationProcessor 'com.jakewharton:butterknife-compiler:8.4.0'
    compile 'com.koushikdutta.ion:ion:2.1.9'
    testCompile 'junit:junit:4.12'
}

和我使用的活动：

public class MainActivity extends AppCompatActivity{

    @BindView(R.id.bottomBar) BottomNavigationView bottomNavigationView;
    private Fragment fragment;
    private FragmentManager fragmentManager;
    private FragmentTransaction transaction;
    private RestService service;
    @BindView(R.id.toolbar) Toolbar toolbar;
    private List<DummyJSONModel> modelFromJson = new ArrayList<>();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.bind(this);
        ButterKnife.setDebug(true);
        setupToolbar();
        setupBottomBar();
        service = new RestService();
        DummyJSONModelWrapper wrapper = (DummyJSONModelWrapper)getIntent().getSerializableExtra(AppConstants.INTENT_KEY);
        modelFromJson = wrapper.getDummyJSONModel();
    }

    private void setupToolbar() {
        toolbar.setTitle(getString(R.string.app_name));
        setSupportActionBar(toolbar);
    }
}

有时我会收到NullpointereException，错误消息为：

cannot android.support.v7.widget.Toolbar.setTitle() on a null object reference

为什么会发生这种情况？