Fixed code to correctly unregister a Collector that is not registered (#795)

Signed-off-by: Doug Hoard <[email protected]>

Author: dhoard

simpleclient/src/main/java/io/prometheus/client/CollectorRegistry.java

@@ -80,8 +80,10 @@ private void assertNoDuplicateNames(Collector m, List<String> names) {
   public void unregister(Collector m) {
     synchronized (namesCollectorsLock) {
       List<String> names = collectorsToNames.remove(m);
-      for (String name : names) {
-        namesToCollectors.remove(name);
+      if (names != null) {
+        for (String name : names) {
+          namesToCollectors.remove(name);
+        }
       }
     }
   }

simpleclient/src/test/java/io/prometheus/client/CollectorRegistryTest.java

@@ -42,6 +42,11 @@ public void testUnregister() {
     assertEquals("c", mfs.get(0).name);
   }
 
+  @Test
+  public void testUnregisterNonRegistered() {
+    registry.unregister(new MyCollector());
+  }
+
   @Test
   public void testClear() {
     Collector g = Gauge.build().name("g").help("h").register(registry);