
**Apache Airflow version**: 2.0.1


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): NA

**Environment**: Windows WSL2 (Ubuntu) Local

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release): Ubuntu 18.04
- **Kernel** (e.g. `uname -a`): Linux d255bce4dcd5 5.4.72-microsoft-standard-WSL2
- **Install tools**: Docker -compose
- **Others**:

**What happened**:
I have a dag with tasks:
Task1 - Get Date
Task 2 - Get data from Api call (Have set retires to 3)
Task 3 - Load Data

Task 2 had failed after three attempts. I am unable to clear the task Instance and get the below error in UI. 

[Dag Code](https://github.com/anilkulkarni87/airflow-docker/blob/master/dags/covidNyDaily.py)

```
Python version: 3.8.7
Airflow version: 2.0.1rc2
Node: d255bce4dcd5
-------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/www/auth.py", line 34, in decorated
    return func(*args, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/www/decorators.py", line 60, in wrapper
    return f(*args, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/www/views.py", line 1547, in clear
    return self._clear_dag_tis(
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/www/views.py", line 1475, in _clear_dag_tis
    count = dag.clear(
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/utils/session.py", line 65, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/dag.py", line 1324, in clear
    clear_task_instances(
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/taskinstance.py", line 160, in clear_task_instances
    ti.max_tries = ti.try_number + task_retries - 1
TypeError: unsupported operand type(s) for +: 'int' and 'str'
```

**What you expected to happen**:

I expected to clear the Task Instance so that the task could be scheduled again.

**How to reproduce it**:
1) Clone the repo link shared above
2) Follow instructions to setup cluster.
3) Change code to enforce error in Task 2 
4) Execute and try to clear task instance after three attempts.

![Error pops up when clicked on Clear](https://user-images.githubusercontent.com/10644132/107998258-8e1ee180-6f99-11eb-8442-0c0be5b23478.png)







airflow/models/baseoperator.py
self, content: Any, context: Dict, jinja_env: jinja2.Environment, seen_oids: Set
    ) -> None:
        if id(content) not in seen_oids:
            seen_oids.add(id(content))
            try:
                nested_template_fields = content.template_fields
            except AttributeError:
                
                return
            self._do_render_template_fields(content, nested_template_fields, context, jinja_env, seen_oids)
    def get_template_env(self) -> jinja2.Environment:
        """Fetch a Jinja template environment from the DAG or instantiate empty environment if no DAG."""
        return (
            self.dag.get_template_env()
            if self.has_dag()
            else airflow.templates.SandboxedEnvironment(cache_size=0)
        )  
    def prepare_template(self) -> None:
        """
        Hook that is triggered after the templated fields get replaced
        by their content. If you need your operator to alter the
        content of the file before the template is rendered,
        it should override this method to do so.
        """
    def resolve_template_files(self) -> None: