Fix: trade window init only lists leagues from the last fetched realm

[apYdr6uxv]

Description of the problem being solved:

Each iteration is wiping the accumulator.
Only the last iteration survives.

Bug very hard to notice in practice because:

1. All realms have (AFAICT) the same leagues as PC (which is the default selected in the UI)
2. The UI is self-healing: re-fetches any missing realms when changing selected realm [1]

Steps taken to verify a working solution:

I used debug logs™

Before

callback realm='pc' - Start
keys={}
callback realm='pc' - End
keys={pc=[Standard,Hardcore,Ruthless,Hardcore Ruthless]}

callback realm='sony' - Start
keys={pc=[Standard,Hardcore,Ruthless,Hardcore Ruthless]}
callback realm='sony' - End
keys={sony=[Standard,Hardcore,Ruthless,Hardcore Ruthless]}

callback realm='xbox' - Start
keys={sony=[Standard,Hardcore,Ruthless,Hardcore Ruthless]}
callback realm='xbox' - End
keys={xbox=[Standard,Hardcore,Ruthless,Hardcore Ruthless]}

After

callback realm='xbox' - Start
keys={}
callback realm='xbox' - End
keys={xbox=[Standard,Hardcore,Ruthless,Hardcore Ruthless]}

callback realm='pc' - Start
keys={xbox=[Standard,Hardcore,Ruthless,Hardcore Ruthless]}
callback realm='pc' - End
keys={xbox=[Standard,Hardcore,Ruthless,Hardcore Ruthless]  pc=[Standard,Hardcore,Ruthless,Hardcore Ruthless]}

callback realm='sony' - Start
keys={xbox=[Standard,Hardcore,Ruthless,Hardcore Ruthless]  pc=[Standard,Hardcore,Ruthless,Hardcore Ruthless]}
callback realm='sony' - End
keys={sony=[Standard,Hardcore,Ruthless,Hardcore Ruthless]  xbox=[Standard,Hardcore,Ruthless,Hardcore Ruthless]  pc=[Standard,Hardcore,Ruthless,Hardcore Ruthless]}