https://imgur.com/RDv4oBu

same thing just happened to me. I also noticed that the rail signal is not populating when trying to use the tool. only the chain signal is populating.

Extract the RailSignalPlanner_1.1.0.zip
Delete the zip file
Go into the folder you just created by extracting the zip
Go into the gui folder
Open signal_tool_gui.lua
Change line 84 to read:
event.element.parent.wagon_length_textfield.text = tostring(event.element.slider_value)
Change line 86 to read:
event.element.parent.parent.train_length_flow.train_length_textfield.text = tostring(7*slider_value - 1)

Should get it working.

Yes, Error on Chang:

Error while running event RailSignalPlanner::on_gui_value_changed (ID 87)
RailSignalPlanner/gui/signal_tool_gui.lua:84: bad argument #1 of 2 to 'newindex' (string expected, got number)
stack traceback:
[C]: in function '__newindex'
__RailSignalPlanner/gui/signal_tool_gui.lua:84: in function <RailSignalPlanner/gui/signal_tool_gui.lua:80>

I updated the lines now crashing my game did not resolve issue.

Fixed in 1.1.1