Fixed wrapper existence check in Receiver constructor

[lionng429]

• as mentioned in Wrapper existence check doesn't work correctly #17, wrapper existence check does not work for wrapperId pointing to Receiver's parent wrapper. Moving the logic back to componentDidMount solves this issue.

it('should not throw if wrapperId is given and element elements having that id exists', () => {
    expect(() => mount(
        <div id="customId">
          <Receiver
            wrapperId="customId"
            onDragEnter={emptyFn}
            onDragOver={emptyFn}
            onDragLeave={emptyFn}
            onFileDrop={emptyFn}
          />
        </div>
    )).not.toThrow();
});