Sketch Welcome Screen
Small bug when Sketch "Welcome" screen is shown on app start.

**Expected behavior:**

Rendering should start normally.

**Observed behavior:**

* `TypeError: null is not an object (evaluating 'context.document.currentPage')`
* Sketch shows welcome screen, but does not render document

**How to reproduce:**

Install Sketch and don't check off "show on every launch" on the "Welcome" screen.

**Sketch version:**

43.2
