fix: throw helpful exception when rule has wrong return type

lib/linter/linter.js

@@ -1119,6 +1119,10 @@ function runRules(sourceCode, configuredRules, ruleMapper, parserName, languageO
             };
         }
 
+        if (typeof ruleListeners === "undefined" || ruleListeners === null) {
+            throw new Error(`The create() function for rule '${ruleId}' did not return an object.`);
+        }
+
         // add all the selectors from the rule as listeners
         Object.keys(ruleListeners).forEach(selector => {
             const ruleListener = timing.enabled

tests/lib/linter/linter.js

@@ -7000,6 +7000,22 @@ var a = "test2";
 
             assert(ok);
         });
+
+        it("should throw when rule's create() function does not return an object", () => {
+            const config = { rules: { checker: "error" } };
+
+            linter.defineRule("checker", () => null); // returns null
+
+            assert.throws(() => {
+                linter.verify("abc", config, filename);
+            }, "The create() function for rule 'checker' did not return an object.");
+
+            linter.defineRule("checker", () => {}); // returns undefined
+
+            assert.throws(() => {
+                linter.verify("abc", config, filename);
+            }, "The create() function for rule 'checker' did not return an object.");
+        });
     });
 
     describe("Custom parser", () => {