If you send event params in WGA.fireEvent(...) there is no need to decode them in OpenWGA. Decoding the params JSON sting leads to removing + signs.

Sample:

WGA.fireEvent("my-event", "my-source", {sample:"hallo + world"})

Inside the event script in Openwga you can read the params via

<tml:eventscript onevent="my-event">
    portletEvent.getParameters().get("sample")
</tml:eventscript>

The result will be

hallo   world

The + sign will be removed and replaced by a space.

We now removed that decoding in OpenWGA.