thing.java

private final Object WAIT_LOCK = new Object();
private int taskCount = 0;

@FunctionalInterface
public interface SafeAsyncTask<E extends Throwable> {
    public void run() throws E;
}

public void safeAsyncTask(final SafeAsyncTask task) {
    Bukkit.getScheduler().runTaskAsynchronously(PLUGIN, () -> {
        synchronized (WAIT_LOCK) {
            ++taskCount;
        }
        try {
            task.run();
        }
        catch (Throwable e) {
            e.printStackTrace();
        }
        finally {
            synchronized (WAIT_LOCK) {
                --taskCount;
                WAIT_LOCK.notifyAll();
            }
        }
    });
}
public void waitForTasks() {
    try {
        synchronized (WAIT_LOCK) {
            while (taskCount > 0) {
                WAIT_LOCK.wait();
            }
        }
    }
    catch (InterruptedException err) {
        err.printStackTrace();   
    }
}