Avoid NPE when plugin cannot be instantiated

When there is a problem creating a wrapper plugin,
we should just skip it -- not crash and burn.

Closes #195.

Thanks to Jonathan Hale for catching this.

Author: ctrueden

src/main/java/org/scijava/plugin/AbstractWrapperService.java

@@ -78,7 +78,7 @@ public boolean supports(final DT data) {
 	private <D extends DT> PT findWrapper(final D data) {
 		for (final PluginInfo<PT> plugin : getPlugins()) {
 			final PT instance = getPluginService().createInstance(plugin);
-			if (instance.supports(data)) return instance;
+			if (instance != null && instance.supports(data)) return instance;
 		}
 		return null;
 	}