block-newsletter component - Validate register_block_type function exists

[seemly]

Installing and activating the plugin threw an error when attempting to call the function "register_block_type".

[mindctrl]

@seemly which version of WP are you using?

[seemly]

@mindctrl I just tried to replicate the issue on my local machine. It seems to be an issue on older versions, so this may not be deemed an issue. I went back to 4.9 and I was then able to replicate the error.

I was just setting up a project on a cloud based coding platform (codeanywhere.com), which auto creates the environment for you. Clearly the default install version of Wordpress needs updating on their end.

I looked through the Atomic Blocks existing codebase and saw that other components had validation for function_exists on register_block_type, so I thought I would just submit a quick PR.