Using mastertickets plugin with already existants tickets cause error

[nextime]

When you try to view an "old" ticket, created before to install the plugin, the ticket_custom table don't have the relative ticket_custom.name = 'blocking' entry, so, in line 56 of the web_ui.py of mastertickets you get an error cause it try to do a replace() on a "None" value for block_ids.

A simple patch that move the line 56 after the "if blocks_ids:" line (57) fix this issue.