error if not all instances initially mounted

[SassNinja]

I'm using hot-reload in combination with webpack-loader

I've placed the same component multiple times on my page but I'm only mounting one of them (dependent on viewport)
When I make changes to the component's file I expect only the mounted component to be updated.

However it seems hot-reload tries to (re)mount all tags of that component on the page what causes the following error:

TypeError: Cannot read property 'unmount' of undefined

Some debugging showed me that oldTag is undefined when it's getting reloaded without being mounted before.
Therefore I'd suggest to simply check if oldTag isn't undefined before trying to unmount it
https://github.com/riot/hot-reload/blob/master/src/index.js#L19

@GianlucaGuarini what do you think?

[GianlucaGuarini]

Thank you for this bug. I will have a look at it. It should be trivial to solve it