Fix exception in linter (issue 315)

lib/src/rules/parameter_assignments.dart

Index: lib/src/rules/parameter_assignments.dart
diff --git a/lib/src/rules/parameter_assignments.dart b/lib/src/rules/parameter_assignments.dart
index a3760a241ce8744fb8945937b9914649bf113376..6188f4e4f44d62f6e7ed9178d02dfbcbd6889cbd 100644
--- a/lib/src/rules/parameter_assignments.dart
+++ b/lib/src/rules/parameter_assignments.dart
@@ -69,11 +69,16 @@ class _Visitor extends SimpleAstVisitor {
 
   @override
   void visitFunctionDeclaration(FunctionDeclaration node) {
-    node.functionExpression.parameters.parameters.forEach((e) {
-      if (node.functionExpression.body.isPotentiallyMutatedInScope(e.element)) {
-        rule.reportLint(e);
-      }
-    });
+    FormalParameterList parameters = node.functionExpression.parameters;
+    if (parameters != null) {
+      // Getter do not have formal parameters.
+      parameters.parameters.forEach((e) {
+        if (node.functionExpression.body
+            .isPotentiallyMutatedInScope(e.element)) {
+          rule.reportLint(e);
+        }
+      });
+    }
   }
 
   @override