flutter - 如何解决意外的空值

Question

我在我的应用程序中添加了一个 float 按钮，同时单击它出现两个文本字段，没有添加 formKey 该按钮有效但是当我使用它来控制字段文本是否为空时if 语句 if (formKey.currentState!.validate()) 在调试控制台中出现此错误

════════ Exception caught by gesture ═══════════════════════════════════════════
Unexpected null value.
═══════════════════════════════════════════════════════════════════════════════

这是 float 按钮的代码

 floatingActionButton: FloatingActionButton(
          onPressed: () {
            if (isBottomSheetShown) {
              if (formKey.currentState!.validate()) {
                Navigator.pop(context);
                isBottomSheetShown = false;
                setState(() {
                  fabIcon = Icons.edit;
                });
              }
            } else {
              scaffoldKey.currentState!.showBottomSheet(
                (context) => Container(
                  color: Colors.grey[100],
                  padding: const EdgeInsets.all(20),
                  child: Form(
                    key: formKey,
                    child: Column(
                      mainAxisSize: MainAxisSize.min,
                      children: [
                        TextField(
                          controller: titleController,
                          decoration: InputDecoration(
                            prefixIcon: Icon(Icons.title),
                            hintText: 'write a title',
                            labelText: 'Title',
                            border: OutlineInputBorder(),
                          ),
                        ),
                        SizedBox(
                          height: 15,
                        ),
                        TextField(
                          controller: timeController,
                          decoration: InputDecoration(
                            prefixIcon: Icon(Icons.watch_later_outlined),
                            hintText: 'write a time',
                            labelText: 'Time',
                            border: OutlineInputBorder(),
                          ),
                          onTap: () {
                            showTimePicker(
                              context: context,
                              initialTime: TimeOfDay.now(),
                            ).then((value) {
                              timeController.text =
                                  value!.format(context).toString();
                              print(value!.format(context));
                            });
                          },
                        ),
                      ],
                    ),
                  ),
                ),
              );
            }
            isBottomSheetShown = true;
            setState(() {
              fabIcon = Icons.add;
            });
          },
          child: Icon(
            fabIcon,
          )),

Answer 1

出现意外的 null 值是因为您在验证之前断言 formKey.currentState 不为 null，因此如果 currentState 等于 null，它将抛出该错误。

我建议在调用验证方法之前先检查是否 formKey.currentState != null。