Do not create new singleton plugin instances

Instead, throw an exception with a hint.

This will hopefully avoid logic errors in downstream code,
which attempt to create new instances of singleton plugins.

Author: ctrueden

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

@@ -80,6 +80,15 @@ public <P extends PT> P getInstance(final Class<P> pluginClass) {
 		return (P) instanceMap.get(pluginClass);
 	}
 
+	// -- PTService methods --
+
+	@Override
+	public <P extends PT> P create(final Class<P> pluginClass) {
+		throw new UnsupportedOperationException(
+			"Cannot create singleton plugin instance. "
+				+ "Use getInstance(Class) instead.");
+	}
+
 	// -- Service methods --
 
 	@Override