attempt to index field 'scrollbar' (a nil value) when signature is enabled

[stefanboca]

Make sure you have done the following

• I have updated to the latest version of blink.cmp
• I have read the README

Bug Description

e3b3fde introduced the following error when signature is enabled:

Error executing vim.schedule lua callback: ...ata/plugins/blink.cmp/lua/blink/cmp/signature/window.lua:157: attempt to index field 'scrollbar' (a nil value)
stack traceback:
	...ata/plugins/blink.cmp/lua/blink/cmp/signature/window.lua:157: in function 'update_position'
	...ata/plugins/blink.cmp/lua/blink/cmp/signature/window.lua:88: in function 'open_with_signature_help'
	.../data/plugins/blink.cmp/lua/blink/cmp/signature/init.lua:16: in function 'callback'
	...ata/plugins/blink.cmp/lua/blink/cmp/sources/lib/init.lua:216: in function 'fn'
	...orwho/data/plugins/blink.cmp/lua/blink/cmp/lib/async.lua:86: in function 'cb'
	...orwho/data/plugins/blink.cmp/lua/blink/cmp/lib/async.lua:44: in function 'resolve'
	...orwho/data/plugins/blink.cmp/lua/blink/cmp/lib/async.lua:167: in function 'resolve_if_completed'
	...orwho/data/plugins/blink.cmp/lua/blink/cmp/lib/async.lua:174: in function 'cb'
	...orwho/data/plugins/blink.cmp/lua/blink/cmp/lib/async.lua:44: in function 'resolve'
	...ns/blink.cmp/lua/blink/cmp/sources/lib/provider/init.lua:158: in function <...ns/blink.cmp/lua/blink/cmp/sources/lib/provider/init.lua:158>

Relevant configuration

signature = {
  enabled = true
}

neovim version

NVIM v0.11.0-dev+1335-g3f1d09bc9 Build type: RelWithDebInfo LuaJIT 2.1.1720049189

blink.cmp version: branch, tag, or commit

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