allow non-existent dynamic components to be destroyed (#1660)

src/compile/nodes/Component.ts

@@ -505,7 +505,7 @@ export default class Component extends Node {
 
 		if (this.compiler.options.nestedTransitions) {
 			block.builders.outro.addLine(
-				`${name}._fragment.o(#outrocallback);`
+				`if (${name}) ${name}._fragment.o(#outrocallback);`
 			);
 		}
 	}

test/runtime/samples/dynamic-component-destroy-null/_config.js

@@ -0,0 +1,13 @@
+export default {
+	data: {
+		x: true
+	},
+
+	nestedTransitions: true,
+
+	test(assert, component) {
+		component.set({
+			x: false
+		});
+	}
+};

test/runtime/samples/dynamic-component-destroy-null/main.html

@@ -0,0 +1,3 @@
+{#if x}
+	<svelte:component this={null}/>
+{/if}