GH-249 getAllEngineInstances() can return collection with null elements

(cherry picked from commit 6377e2b)

Author: thurka

visualvm/sampler.truffle/libsrc/org/graalvm/visualvm/sampler/truffle/stagent/Truffle.java

@@ -213,6 +213,7 @@ private Collection<CPUSampler> getAllStackTracesInstances() {
         Collection<Engine> all = getAllEngineInstances();
 
         for (Engine engine : all) {
+            if (engine == null) continue;
             CPUSampler sampler = CPUSampler.find(engine);
 
             if (sampler != null) {
@@ -251,6 +252,7 @@ private Collection<HeapMonitor> getAllHeapHistogramInstances() {
         Collection<Engine> all = getAllEngineInstances();
 
         for (Engine engine : all) {
+            if (engine == null) continue;
             HeapMonitor heapHisto = HeapMonitor.find(engine);
 
             if (heapHisto != null) {
@@ -333,6 +335,7 @@ private void setMode(Mode m) {
         Collection<Engine> all = getAllEngineInstances();
 
         for (Engine engine : all) {
+            if (engine == null) continue;
             CPUSampler sampler = CPUSampler.find(engine);
 
             if (sampler != null) {