lua: fix theme loading when lexer module is not available

The color settings are currently stored in the `vis.lexers` table,
make sure it is not nil even when loading the lexer module (or one
of its dependencies e.g. lpeg) failed.

lua/plugins/number-inc-dec.lua

@@ -1,7 +1,7 @@
 -- increment/decrement number in dec/hex/oct format
 local lexer = vis.lexers
 local lpeg = vis.lpeg
-if not lexer or not lpeg then return end
+if not lexer.load or not lpeg then return end
 
 local Cp = lpeg.Cp()
 local dec_num = lpeg.S('+-')^-1 * lexer.dec_num

lua/plugins/textobject-lexer.lua

@@ -4,7 +4,7 @@ local MAX_CONTEXT = 32768
 
 vis:textobject_new("ii", function(win, pos)
 
-	if win.syntax == nil or not vis.lexers then
+	if not win.syntax or not vis.lexers.load then
 		return nil
 	end
 

lua/vis-std.lua

@@ -14,7 +14,7 @@ vis:option_register("theme", "string", function(name)
 		require(theme)
 	end
 
-	if vis.lexers then vis.lexers.lexers = {} end
+	vis.lexers.lexers = {}
 
 	for win in vis:windows() do
 		win:set_syntax(win.syntax)
@@ -38,9 +38,9 @@ vis:option_register("horizon", "number", function(horizon)
 end, "Number of bytes to consider for syntax highlighting")
 
 vis.events.subscribe(vis.events.WIN_HIGHLIGHT, function(win)
-	if win.syntax == nil or vis.lexers == nil then return end
+	if not win.syntax or not vis.lexers.load then return end
 	local lexer = vis.lexers.load(win.syntax, nil, true)
-	if lexer == nil then return end
+	if not lexer then return end
 
 	-- TODO: improve heuristic for initial style
 	local viewport = win.viewport

lua/vis.lua

@@ -113,6 +113,8 @@ vis.module_exist = function(vis, name)
 	return false
 end
 
+vis.lexers = {}
+
 if not vis:module_exist('lpeg') then
 	vis:info('WARNING: could not find lpeg module')
 elseif not vis:module_exist('lexer') then
@@ -235,7 +237,6 @@ vis.events = events
 vis.types.window.set_syntax = function(win, syntax)
 
 	local lexers = vis.lexers
-	if not lexers then return false end
 
 	win:style_define(win.STYLE_DEFAULT, lexers.STYLE_DEFAULT or '')
 	win:style_define(win.STYLE_CURSOR, lexers.STYLE_CURSOR or '')
@@ -256,6 +257,7 @@ vis.types.window.set_syntax = function(win, syntax)
 		return true
 	end
 
+	if not lexers.load then return false end
 	local lexer = lexers.load(syntax)
 	if not lexer then return false end