Android Firebase child()报pathString参数为null错误如何解决

开发Android应用实现菜谱数据更新功能时触发运行时崩溃，报错信息为 Can't pass null for argument 'pathString' in child()，崩溃定位到代码行：
databaseReference = FirebaseDatabase.getInstance().getReference("Recipe_Credentials").child(key);
提示传入child()方法的key参数为null。

崩溃由代码执行顺序错误导致：

• 成员变量key声明后未赋值时默认值为null
• 原代码在onCreate生命周期中，先执行Firebase数据库引用初始化、调用.child(key)，此时还未从跳转Intent的Bundle中提取并给key赋值
• Firebase Realtime Database的child()方法不接受null作为路径参数，直接触发空指针异常

• 调整onCreate内代码执行顺序：先解析Intent携带的Bundle参数，完成key的赋值后，再初始化对应路径的DatabaseReference
• 增加参数合法性校验：如果Bundle中未携带合法的key值，做兜底异常处理，避免崩溃

修正后的核心实现代码如下：

public class Update_Activity extends AppCompatActivity {

    ImageView update_Recipe_Image;
    EditText update_Recipe_Name, update_Recipe_Description, update_Recipe_Price;
    Button updatebutton, selectImageButton;
    String key, oldimageUrl;
    String imageUrl;
    DatabaseReference databaseReference;
    StorageReference storageReference;
    String recipename, recipedescription, recipeprice;

    private Uri filePath;
    private final int PICK_IMAGE_REQUEST = 10;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_update);

        update_Recipe_Image = (ImageView) findViewById(R.id.update_Recipe_Image);
        update_Recipe_Name = (EditText) findViewById(R.id.update_Recipe_Name);
        update_Recipe_Description = (EditText) findViewById(R.id.update_Recipe_Description);
        update_Recipe_Price = (EditText) findViewById(R.id.update_Recipe_Price);
        updatebutton = (Button) findViewById(R.id.updatebutton);
        selectImageButton = (Button) findViewById(R.id.selectImageButton);

        // 优先解析Intent传递的参数
        Bundle bundle = getIntent().getExtras();
        if (bundle != null){
            Glide.with(Update_Activity.this)
                    .load(bundle.getString("oldimageUrl"))
                    .into(update_Recipe_Image);
            update_Recipe_Name.setText(bundle.getString("recipeNameKey"));
            update_Recipe_Description.setText(bundle.getString("recipeDescriptionKey"));
            update_Recipe_Price.setText(bundle.getString("recipePriceKey"));
            key = bundle.getString("key");
            oldimageUrl = bundle.getString("oldimageUrl");
        }

        // 校验key合法性后再初始化数据库引用
        if (key == null || key.isEmpty()) {
            Toast.makeText(this, "菜谱参数异常，无法打开编辑页", Toast.LENGTH_SHORT).show();
            finish();
            return;
        }
        databaseReference = FirebaseDatabase.getInstance().getReference("Recipe_Credentials").child(key);

        selectImageButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                chooseImage();
            }
        });

        updatebutton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                recipename = update_Recipe_Name.getText().toString().trim();
                recipedescription = update_Recipe_Description.getText().toString().trim();
                recipeprice = update_Recipe_Price.getText().toString().trim();
                uploadImage();
            }
        });
    }
}

触发崩溃时Logcat输出的核心栈信息如下：

Caused by: java.lang.NullPointerException: Can't pass null for argument 'pathString' in child()
        at com.google.firebase.database.DatabaseReference.child(DatabaseReference.java:96)
        at com.example.universityofloralaicafeteria.Admin.Update_Activity.onCreate(Update_Activity.java:62)
        at android.app.Activity.performCreate(Activity.java:7183)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1220)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2910)

栈信息明确指向onCreate方法中调用child()的代码行，和代码顺序导致的key为null问题完全匹配。

内容的提问来源于stack exchange，提问作者Jabbar Kakar