languageclient: check for existence of 'result' key

fixes #1861

autoload/airline/extensions/languageclient.vim

@@ -1,4 +1,6 @@
 " MIT License. Copyright (c) 2013-2018 Bjorn Neergaard, hallettj et al.
+" This comes from the Languageclient plugin:
+" https://github.com/autozimu/LanguageClient-neovim
 " vim: et ts=2 sts=2 sw=2
 
 scriptencoding utf-8
@@ -25,8 +27,11 @@ function! s:languageclient_refresh()
 endfunction
 
 function! s:record_diagnostics(state)
-  let result = json_decode(a:state.result)
-  let s:diagnostics = result.diagnostics
+  " The returned message might not have the 'result' key
+  if has_key(a:state, 'result')
+    let result = json_decode(a:state.result)
+    let s:diagnostics = result.diagnostics
+  endif
   call s:languageclient_refresh()
 endfunction