Ignore non-ObjectExpression's in propTypeHandler.

If `propTypes` is a member expression, the handler used to throw the
error

TypeError: Cannot read property 'length' of undefined
  at NodePath.each (/react-docgen/node_modules/ast-types/lib/path.js:82:25)
  at amendPropTypes (/react-docgen/dist/handlers/propTypeHandler.js:69:26)
  at propTypeHandler (/react-docgen/dist/handlers/propTypeHandler.js:103:3)

Author: fkling

src/handlers/__tests__/propTypeHandler-test.js

@@ -202,5 +202,20 @@ describe('propTypeHandler', () => {
     definition = statement('class Foo {}');
     expect(() => propTypeHandler(documentation, definition))
       .not.toThrow();
+
+    definition = statement('function Foo() {}');
+    expect(() => propTypeHandler(documentation, definition))
+      .not.toThrow();
+
+    definition = expression('() => {}');
+    expect(() => propTypeHandler(documentation, definition))
+      .not.toThrow();
+  });
+
+  // This case is handled by propTypeCompositionHandler
+  it('does not error if propTypes is a member expression', () => {
+    var definition = expression('{propTypes: Foo.propTypes}');
+    expect(() => propTypeHandler(documentation, definition))
+      .not.toThrow();
   });
 });

src/handlers/propTypeHandler.js

@@ -34,6 +34,10 @@ function isPropTypesExpression(path) {
 }
 
 function amendPropTypes(documentation, path) {
+  if (!types.ObjectExpression.check(path.node)) {
+    return;
+  }
+
   path.get('properties').each(function(propertyPath) {
     switch (propertyPath.node.type) {
       case types.Property.name: