I'm trying to get data from firebase but flutter is showing null safety error on StreamBuilder

I’m using StreamBuilder and inside that, I have got a list of tasks which takes input from the snapshots and display the data accordingly, but I’m having a null safety issue when dealing with snapshot data. Here’s what the code looks like:

StreamBuilder<List<Task>>(
  stream: DatabaseService()
      .getCompletedTasks(orderName),
  builder: (context, snapshot) {
    List<Task> completedTasks =
        snapshot.data!;
    return snapshot.data!.isEmpty
        ? Container(
            alignment:
                Alignment.center,
            child: Text(
              "You don't have any completed tasks.",
              style: TextStyle(
                fontSize: 20,
                fontFamily:
                    "Roboto",
                color: Colors.black,
              ),
            ),
          )
        : Container(
            child: Flexible(
              flex: 0,
              child: ListView
                  .separated(
                physics:
                    const NeverScrollableScrollPhysics(),
                shrinkWrap: true,
                separatorBuilder:
                    (context,
                            index) =>
                        const SizedBox(
                  height: 5,
                ),
                itemCount:
                    completedTasks
                        .length,
                itemBuilder:
                    (context,
                        index) {
                  return Dismissible(
                    background:
                        Container(
                      color: Colors
                          .red,
                      child:
                          const Align(
                        alignment:
                            Alignment
                                .centerLeft,
                        child:
                            Padding(
                          padding:
                              EdgeInsets.all(
                                  15.0),
                          child:
                              Icon(
                            Icons
                                .delete,
                            color: Colors
                                .white,
                          ),
                        ),
                      ),
                    ),
                    secondaryBackground:
                        Container(
                      color: Colors
                          .red,
                      child:
                          const Align(
                        alignment:
                            Alignment
                                .centerRight,
                        child:
                            Padding(
                          padding:
                              EdgeInsets.all(
                                  15.0),
                          child:
                              Icon(
                            Icons
                                .delete,
                            color: Colors
                                .white,
                          ),
                        ),
                      ),
                    ),
                    key:
                        UniqueKey(),
                    onDismissed:
                        (direction) {
                      DatabaseService()
                          .deleteTask(
                              completedTasks[index]
                                  .id);
                      AwesomeNotifications()
                          .cancelSchedule(
                              completedTasks[index]
                                  .id);
                    },
                    child: Item(
                      task:
                          completedTasks[
                              index],
                    ),
                  );
                },
              ),
            ),
          );
  }),

Everything works fine but just null safety error pops up for a short amount of time.