Summary:
The mod crashes with a non-recoverable error when the event on_player_selected_area is triggered, due to an invalid implicit API call in the graph-building logic.

Error Details:

Error Message:
"implicitly API call when implicitly was invalid."

Location in Code:
RailSignalPlanner/scripts/logic/build_graph.lua:169 inside function revive_ghost_signals.

Triggering Event:
on_player_selected_area (Event ID 57).

Impact:
Causes a multiplayer server to fail and shut down, deleting the active scenario.

Stack Trace Highlights:

[C]: in function '_index'

build_graph.lua:169 in revive_ghost_signals

build_graph.lua:180 in build_graph

control.lua:126 (or control.lua:68) in build_signals

control.lua:172 in on_player_selected_area

Possible Causes:

The mod attempts to call a Factorio API method that is not available or is in an invalid state when processing ghost signals.

A race condition or invalid internal state during area selection and signal graph construction.

The revive_ghost_signals function may be accessing game entities or properties that no longer exist or are not fully initialized.

Reproduction Steps (Inferred):

Play in multiplayer (or single-player) with Rail Signal Planner enabled.

Use the mod’s area selection tool to select a region for signal planning.

The error occurs immediately upon selection, crashing the game session.

Suggested Fixes for Developer:

Validate API availability before implicit calls in revive_ghost_signals.

Add defensive checks for entity validity and game state in the graph-building process.

Review the _index metamethod usage at line 169 in build_graph.lua.