Fix crash with prefer-type-error rule

Fixes #82

Author: sindresorhus

rules/prefer-type-error.js

@@ -58,10 +58,7 @@ const throwsErrorObject = node =>
 	node.argument.callee.type === 'Identifier' &&
 	node.argument.callee.name === 'Error';
 
-const isLone = node =>
-	node.parent !== null &&
-	node.parent.body !== null &&
-	node.parent.body.length === 1;
+const isLone = node => node.parent && node.parent.body && node.parent.body.length === 1;
 
 const isTypecheckingMemberExpression = (node, callExpression) => {
 	if (isTypecheckingIdentifier(node.property, callExpression, true)) {
@@ -106,7 +103,7 @@ const create = context => {
 		ThrowStatement: node => {
 			if (throwsErrorObject(node) &&
 				isLone(node) &&
-				node.parent.parent !== null &&
+				node.parent.parent &&
 				isTypechecking(node.parent.parent)) {
 				context.report({
 					node,

test/prefer-type-error.js

@@ -194,6 +194,13 @@ ruleTester.run('prefer-type-error', rule, {
 		}`,
 		`if (!_.isFudge(x)) {
 			throw new Error('x is no fudge!');
+		}`,
+		// Should not crash:
+		`switch (something) {
+			case 1:
+				break;
+			default:
+				throw new Error('Unknown');
 		}`
 	],
 	invalid: [