You're probably getting this warning because you're accessing methods on a component before it gets mounted. This will be soon be an unsupported use case.
The component that gets created by invoking the convenience constructor MyComponent(props, children) or using the JSX syntax <MyComponent /> is not guaranteed to be the instance that actually gets mounted. It's a description of what the actual mounted instance should look like.
Anti-pattern:
The component also doesn't have access to state or other important information such as the final props. As a consequence you shouldn't be calling custom methods on this object.
var MyComponent = React.createClass({