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[Rawfb Wayland demo](https://github.com/Immediate-Mode-UI/Nuklear/tree/master/demo/rawfb/wayland) is currently broken as described in issue https://github.com/Immediate-Mode-UI/Nuklear/issues/806. This PR fixes it by assigning the return value of [`nk_rawfb_init`](https://github.com/Immediate-Mode-UI/Nuklear/blob/master/demo/rawfb/wayland/main.c#L519) to the uninitialized `nk_wayland_ctx.rawfb`.

Rawfb Wayland Demo Is Broken

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## Problem I compiled the [rawfb Wayland demo](https://github.com/Immediate-Mode-UI/Nuklear/tree/master/demo/rawfb/wayland) from the master branch without problems. However, executing the demo binary results in this error message: ``` demo: ../../../nuklear.h:20668: nk_begin_titled: Assertion `ctx'...