synchronizing both access to the timer object. Thanks Romain.

org.sat4j.core/src/main/java/org/sat4j/minisat/core/Solver.java

@@ -1830,10 +1830,14 @@ public class Solver<D extends DataStructureFactory>
 
         cancelUntil(0);
         cancelLearntLiterals(learnedLiteralsLimit);
-        if (!global && this.timeBasedTimeout && this.timer != null) {
+        if (!global && this.timeBasedTimeout) {
+            synchronized (this) {
+                if (this.timer != null) {
                     this.timer.cancel();
                     this.timer = null;
                 }
+            }
+        }
         this.slistener.end(status);
         if (!this.undertimeout) {
             String message = " Timeout (" + this.timeout