Fix undefined story data

[jacobcoughenour]

I was getting this error in my storybook for a svelte project:

TypeError: Cannot use 'in' operator to search for 'parameters' in undefined
    at renderTheme (vendors~main.44b08c472af3b68560ea.bundle.js:204953)

I added a check to see if the current story data is undefined before checking if it contains the "parameters" key.

I'm not sure why getCurrentStoryData() is returning null, but now renderTheme() can handle it.

[jacobcoughenour]

I figured out that I was calling load() twice in my storybook config. Once I fixed that, the error is gone.

Maybe a warning should be added if getCurrentStoryData() returns null?

[hipstersmoothie]

🚀 PR was released in v0.3.1 🚀