React testing error "Cannot read get property of undefined"

Question

I am trying to test the function searchTrigger in my CardMain component.

export default class CardMain extends Component {

    state = {
        Pools : [],
        loading: false,
    }

    componentDidMount(){
        axios.get('/pools')
        .then (res => {
                //console.log(res.data.data);
                this.setState({
                    Pools: res.data.data,
                    loading: true,
                    message: "Loading..."
                },()=>{
                    if (res && isMounted){
                        this.setState({
                            loading: false
                        });
                    }
                })
            }
        )
        .catch(err=>{
            console.log(err.message);
        })
    }

    


    // the function is for search method
    // upon search, this function is called and the state of the pools is changed
    searchTrigger = (search) => {

        Search = search.toLowerCase();

        SearchList = this.state.Pools.filter((e)=> {
            if (e.name.toLowerCase().includes(Search)){
                this.setState({
                    loading: false
                })
                return e
            } 
        })

        if (SearchList.length === 0){
            this.setState({
                loading: true,
                message: "No pools found"
            })
        }
    }


    render() {
        return (
            <div>
                <Searchbar trigger={this.searchTrigger}/>
                { this.state.loading ?
                 <div className="d-flex justify-content-center">{this.state.message}</div>   
                :<div>
                   {Search === "" ? <Card1 pools={this.state.Pools}/> : <Card1 pools={SearchList}/> }
                </div>
                }
            </div>
        )
    }
}

The function searchTrigger is passed to another class component called Searchbar which basically displays the search bar. Upon searching something, the function searchTrigger is called and the searched value is passed as an argument to this function.

So, I am trying to test this function and I am new to react and testing. I found some examples online and tried a simple testing whether the function is called or not. My CardMain.test.js code looks like this:

describe("callback function test", ()=> {
    it("runs it", () => {
        //const spy = jest.spyOn(CardMain.prototype,"searchTrigger");
        const cardmain = shallow(<CardMain/>)
        const spy = jest.spyOn(cardmain.instance(), "searchTrigger");
        expect(spy).toHaveBeenCalled()
    })
});

I get the TypeError: Cannot read property 'get' of undefined pointing to the axios.get("/pools") in the CardMain component inside componentDidMount. axios is being imported from another component api.js which creates the instance of axios using axios.create. I have no idea what the problem is. I am very new to react. I have absolutely no idea, how do I test these components? Could somebody help me?

Update:

So, i tried mocking axios call:
let Wrapper;
beforeEach(() => {
    Wrapper = shallow( <CardMain/>);
  });


describe("Card Main", ()=> {
    it("returns data when called", done => {
        let mock = new MockAdapter(axios);
        const data = [{
            name: "Test",
            response: true
        }];
        mock.onGet('My_URL')
        .reply(200,data);
        const instance = Wrapper.instance();
        instance.componentDidMount().then(response => {
            expect(response).toEqual(data);
            done();
        });
    });
});

It says "cannot read property .then of undefined"