[api] avoid a crash on non UTF-8 files

src/api/app/helpers/webui/webui_helper.rb

@@ -305,11 +305,11 @@ def elide_two(text1, text2, overall_length = 40, mode = :middle)
   end
 
   def force_utf8_and_transform_nonprintables(text)
+    text.force_encoding('UTF-8')
     unless text.valid_encoding?
       text = 'The file you look at is not valid UTF-8 text. Please convert the file.'
     end
     # Ged rid of stuff that shouldn't be part of PCDATA:
-    text.force_encoding('UTF-8')
     return text.gsub(/([^a-zA-Z0-9&;<>\/\n \t()])/) do
       if $1[0].getbyte(0) < 32
         ''