[Fix] shallow/mount: throw an explicit error when state is null/undefined

Author: ljharb

packages/enzyme-test-suite/test/ReactWrapper-spec.jsx

@@ -161,6 +161,7 @@ describeWithDOM('mount', () => {
   </div>
 </Foo>
       `.trim());
+      expect(() => wrapper.state('key')).to.throw('ReactWrapper::state("key") requires that `state` not be `null` or `undefined`');
     });
 
     describeIf(is('>= 16.3'), 'uses the isValidElementType from the Adapter to validate the prop type of Component', () => {

packages/enzyme-test-suite/test/ShallowWrapper-spec.jsx

@@ -153,6 +153,7 @@ describe('shallow', () => {
   null
 </div>
       `.trim());
+      expect(() => wrapper.state('key')).to.throw('ShallowWrapper::state("key") requires that `state` not be `null` or `undefined`');
     });
   });
 

packages/enzyme/src/ReactWrapper.js

@@ -678,6 +678,9 @@ class ReactWrapper {
     }
     const _state = this.single('state', () => this.instance().state);
     if (typeof name !== 'undefined') {
+      if (_state == null) {
+        throw new TypeError(`ReactWrapper::state("${name}") requires that \`state\` not be \`null\` or \`undefined\``);
+      }
       return _state[name];
     }
     return _state;

packages/enzyme/src/ShallowWrapper.js

@@ -916,6 +916,9 @@ class ShallowWrapper {
     }
     const _state = this.single('state', () => this.instance().state);
     if (typeof name !== 'undefined') {
+      if (_state == null) {
+        throw new TypeError(`ShallowWrapper::state("${name}") requires that \`state\` not be \`null\` or \`undefined\``);
+      }
       return _state[name];
     }
     return _state;