问题是因为您在检查标题和说明是否为空时错误地访问了变量，如下所示：

if (!TextUtils.isEmpty(title) && !TextUtils.isEmpty(description_val)) {

应该是：

public void createPost() {
    String title_val = title.getText().toString().trim();
    String post_description_val = description.getText().toString().trim();
    String user_id=mAuth.getCurrentUser().getUid();
    if (!title_val.isEmpty() && !post_description_val.isEmpty()) {
        progressBar.setVisibility(View.VISIBLE);
        Map<String, String> posts = new HashMap<>();
        posts.put("description",description_val);
        posts.put("title", title);
        posts.put("user_id", user_id);
        progressBar.setVisibility(View.GONE);
        db.collection("posts")
                .add(posts)
                .addOnSuccessListener(new OnSuccessListener<DocumentReference>() {
                    @Override
                    public void onSuccess(DocumentReference documentReference) {
                        Toast.makeText( Post.this,"Success", Toast.LENGTH_SHORT).show();
                    }
                })
                .addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                        Log.w( TAG,"Error adding document", e);
                    }
                });
    }

}

编辑：

出于测试目的，您的规则应如下所示，稍后您需要更新它们以使其更安全：