plugin manager: fix plugins with multiple dependencies

If one plugin has multiple dependencies this would throw a concurrent
modification exception due to iterating the successors after removing
the edge

Fill out javadoc for topologicalSort

Co-authored-by: Adam <Adam@sigterm.info>

runelite-client/src/main/java/net/runelite/client/plugins/PluginManager.java

@@ -631,11 +631,14 @@ public class PluginManager
 	/**
 	 * Topologically sort a graph. Uses Kahn's algorithm.
 	 *
-	 * @param graph
-	 * @param <T>
-	 * @return
+	 * @param graph - A directed graph
+	 * @param <T>   - The type of the item contained in the nodes of the graph
+	 * @return - A topologically sorted list corresponding to graph.
+	 * <p>
+	 * Multiple invocations with the same arguments may return lists that are not equal.
 	 */
-	private <T> List<T> topologicalSort(Graph<T> graph)
+	@VisibleForTesting
+	static <T> List<T> topologicalSort(Graph<T> graph)
 	{
 		MutableGraph<T> graphCopy = Graphs.copyOf(graph);
 		List<T> l = new ArrayList<>();
@@ -650,7 +653,7 @@ public class PluginManager
 
 			l.add(n);
 
-			for (T m : graphCopy.successors(n))
+			for (T m : new HashSet<>(graphCopy.successors(n)))
 			{
 				graphCopy.removeEdge(n, m);
 				if (graphCopy.inDegree(m) == 0)