我正在嘗試將自定義圖像設(shè)置到我的標(biāo)題欄中。運(yùn)行以下代碼返回以下內(nèi)容：java.lang.RuntimeException: Unable to start activity ComponentInfo{com.me.project/com.me.project.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.Toolbar.setTitle(int)' on a null object referencetoolbar一片空白。為什么？R.id.app_toolbar這里不為空。主活動(dòng).java：protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        Toolbar toolbar = findViewById(R.id.app_toolbar);        toolbar.setTitle(R.string.app_name);        toolbar.setLogo(R.drawable.app_icon);        setSupportActionBar(toolbar);        FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();        fragmentTransaction.replace(R.id.container, new HomeScreenFragment());        fragmentTransaction.commit();    }活動(dòng)_main.xml：<?xml version="1.0" encoding="utf-8"?><android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:id="@+id/main_activity"    xmlns:tools="http://schemas.android.com/tools"    android:orientation="vertical"    android:layout_width="match_parent"    android:layout_height="match_parent"    tools:context=".MainActivity">    <include        android:id="@+id/app_toolbar"        layout="@layout/app_toolbar"/>    <FrameLayout        android:id="@+id/container"        android:orientation="vertical"        android:layout_width="match_parent"        android:layout_height="match_parent">    </FrameLayout></android.support.constraint.ConstraintLayout>app_toolbar.xml：<?xml version="1.0" encoding="utf-8"?><android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"    android:id="@+id/app_toolbar"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:minHeight="?attr/actionBarSize"    android:background="?attr/colorPrimary"    ></android.support.v7.widget.Toolbar>我看了這個(gè)，無法解決問題。