Unhandled Rejection (Error): [object Object] from Webpack in React?

Question

React

After executing this function:

async addApp(){
        this.setState(produce(draft => {
            draft.preview.button.loading = true;
        }))
        let apps;
        await apiCall('me').then(res => {
            apps = res.data.preferences.apps + (res.data.preferences.apps === '' ? '' : ';') + this.state.preview.appId;
        })
        await apiCall('apps/setapps',{apps}).then(res => {
        this.getPreview(this.state.preview.appId)
            }).catch(err => {
                console.log(err)
            })
    }

An error pops up: Unhandled Rejection (Error): [object Object]
And the error is not from React, but from Webpack.
What could be the reason?