Fix crash with get_keywords_color

(cherry picked from commit 8d993f635776cb5151bba159939700e490576593)

scene/gui/text_edit.cpp

@@ -4457,6 +4457,8 @@ bool TextEdit::has_keyword_color(String p_keyword) const {
 }
 
 Color TextEdit::get_keyword_color(String p_keyword) const {
+
+	ERR_FAIL_COND_V(!keywords.has(p_keyword), Color())
 	return keywords[p_keyword];
 }