Null pointer exception after "Error: current state is not a legal state"

[Calvin-L]

Null pointer exception after "Error: current state is not a legal state"

[lemmy]

This bug is triggered when TLC is run with -generateSpecTE or -dumpTrace options (the former is the default). It attempts to serialize a counterexample that includes states where some variables are undefined (null). It is straightforward (see below) to make -generateSpecTE, -dumpTrace, and similar options as no-ops if the counterexample contains non-good states. However, I assume that some users (particularly tools) of -dumpTrace json would prefer that TLC makes a best effort to serialize even non-good counterexamples. Thoughts?

diff --git a/tlatools/org.lamport.tlatools/src/tlc2/tool/ModelChecker.java b/tlatools/org.lamport.tlatools/src/tlc2/tool/ModelChecker.java
index 31720755f..e454ba019 100644
--- a/tlatools/org.lamport.tlatools/src/tlc2/tool/ModelChecker.java
+++ b/tlatools/org.lamport.tlatools/src/tlc2/tool/ModelChecker.java
@@ -150,7 +150,7 @@ public class ModelChecker extends AbstractChecker
                 if (result != EC.NO_ERROR)
                 {
                     report("exiting, because init failed");
-                    if (this.errState != null) {
+                    if (this.errState != null && tool.isGoodState(this.errState)) {
                         tool.checkPostConditionWithCounterExample(new CounterExample(errState));
                     } else {
                         tool.checkPostCondition();

(Similar check would have to be added for counterexamples with length > 1)