kernel: Fix nasty error.

st.seamless implementation broken the invariant:
"when there was no context on entry to ST,
there should be no context when leaving ST".

This meant, error handling was "switched off" whenever there was error
in a promises' then: or catch: block (as promises' handlers were
called using st.seamless - there's a good reason for that -
there may be (and should be) .catch somewhere down the chain).

st.seamless is fixed to return thisContext to null on leave
when it was null on entry.

CHANGELOG

@@ -1,3 +1,11 @@
+12 Apr 2020 - Release 0.25.3
+===================================
+
+* Fix error handling broken by errors happening in Promises.
+
+Commits: https://lolg.it/amber/amber/commits/0.25.3
+
+
 11 Apr 2020 - Release 0.25.2
 ===================================
 

lang/base/kernel-runtime.js

@@ -355,6 +355,14 @@ define(['./junk-drawer'], function ($goodies) {
 
             var thisContext = null;
 
+            function resultWithNoErrorHandling (worker) {
+                try {
+                    return worker(thisContext);
+                } finally {
+                    thisContext = null;
+                }
+            }
+
             /*
              Runs worker function so that error handler is not set up
              if there isn't one. This is accomplished by unconditional
@@ -368,7 +376,7 @@ define(['./junk-drawer'], function ($goodies) {
                 thisContext = new SmalltalkMethodContext(thisContext, function (ctx) {
                     ctx.fill(null, "seamlessDoIt", {}, globals.UndefinedObject);
                 });
-                var result = worker(thisContext);
+                var result = oldContext == null ? resultWithNoErrorHandling(worker) : worker(thisContext);
                 thisContext = oldContext;
                 return result;
             };