Fixing #2668, add null-checks in on_outro callback for IfBlock

src/compile/render-dom/wrappers/IfBlock.ts

@@ -319,6 +319,9 @@ export default class IfBlockWrapper extends Wrapper {
 		const destroy_old_block = deindent`
 			@group_outros();
 			@on_outro(() => {
+				if (${if_blocks}[${previous_block_index}] === null) {
+					return;
+				}
 				${if_blocks}[${previous_block_index}].d(1);
 				${if_blocks}[${previous_block_index}] = null;
 			});
@@ -430,6 +433,9 @@ export default class IfBlockWrapper extends Wrapper {
 			? deindent`
 				@group_outros();
 				@on_outro(() => {
+					if (${name} === null) {
+						return;
+					}
 					${name}.d(1);
 					${name} = null;
 				});