Fix nullpointer when contraptions stop

src/main/java/com/simibubi/create/content/contraptions/components/structureMovement/render/ContraptionRenderManager.java

@@ -50,13 +50,17 @@ public abstract class ContraptionRenderManager<C extends ContraptionRenderInfo>
 	}
 
 	public void beginFrame(BeginFrameEvent event) {
-		visible.clear();
 
 		renderInfos.int2ObjectEntrySet()
 				.stream()
 				.map(Map.Entry::getValue)
 				.forEach(renderInfo -> renderInfo.beginFrame(event));
 
+		collectVisible();
+	}
+
+	protected void collectVisible() {
+		visible.clear();
 		renderInfos.int2ObjectEntrySet()
 				.stream()
 				.map(Map.Entry::getValue)
@@ -80,6 +84,9 @@ public abstract class ContraptionRenderManager<C extends ContraptionRenderInfo>
 		renderInfos.clear();
 	}
 
+	/**
+	 * Remove all render infos associated with dead/removed contraptions.
+	 */
 	public void removeDeadRenderers() {
 		renderInfos.values().removeIf(ContraptionRenderInfo::isDead);
 	}

src/main/java/com/simibubi/create/content/contraptions/components/structureMovement/render/FlwContraptionManager.java

@@ -80,13 +80,17 @@ public class FlwContraptionManager extends ContraptionRenderManager<RenderedCont
 
 	@Override
 	public void removeDeadRenderers() {
-		renderInfos.values().removeIf(renderer -> {
+		boolean removed = renderInfos.values()
+				.removeIf(renderer -> {
 					if (renderer.isDead()) {
 						renderer.invalidate();
 						return true;
 					}
 					return false;
 				});
+
+		// we use visible in #tick() so we have to re-evaluate it if any were removed
+		if (removed) collectVisible();
 	}
 
 	@Override