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Error-Trace Exploration can't resolve definitions in other modules

See original GitHub issue

Bug repro steps:

Open below specs in the TLA Toolbox (tested in version 1.5.6 on Windows 10 64-bit)
Create a model for the SystemMC spec
Define MaxCounterValue as 5 (or any number you want), ensure deadlock is being checked
Run TLC
When deadlock is reached, open the Error-Trace Exploration window
Add some expression to check (for example, MaxCounterValue)
Click “Explore”; the following error box pops up, unable to find the definition of Null:

------------------------------- MODULE System -------------------------------

EXTENDS
    Naturals

VARIABLES
    Counter

-----------------------------------------------------------------------------

Null == CHOOSE n : n \notin Nat

TypeInvariant ==
    /\ Counter \in Nat \cup {Null}

Init ==
    /\ Counter = Null

FirstIncrement ==
    /\ Counter = Null
    /\ Counter' = 0

Increment ==
    /\ Counter /= Null
    /\ Counter' = Counter + 1

Next ==
    \/ FirstIncrement
    \/ Increment

=============================================================================

------------------------------ MODULE SystemMC ------------------------------

EXTENDS
    Naturals

CONSTANTS
    MaxCounterValue

VARIABLES
    Counter

ASSUME MaxCounterValue \in Nat

-----------------------------------------------------------------------------

S == INSTANCE System

TypeInvariant ==
    /\ S!TypeInvariant

Init ==
    /\ S!Init

Next ==
    IF Counter = S!Null
    THEN S!Next
    ELSE
        /\ Counter < MaxCounterValue
        /\ S!Next

=============================================================================