Fix null pointer exception in references (#236)

CHANGELOG.md

@@ -1,3 +1,10 @@
+## 1.7.2
+
+### Module Migrator
+
+* Fixes a rare crash in certain cases involving reassignments of variables from
+  another module.
+
 ## 1.7.1
 
 * Eliminates invalid warnings when running `--migrate-deps` on a file that

lib/src/migrators/module/references.dart

@@ -638,9 +638,8 @@ class _ReferenceVisitor with RecursiveStatementVisitor, RecursiveAstVisitor {
     var declaration = _scopeForNamespace(namespace).findVariable(node.name);
     if (declaration != null && !_fromForwardRuleInCurrent(declaration)) {
       _variables[node] = declaration;
-      if (declaration.member is VariableDeclaration) {
-        _sources[node] = _declarationSources[declaration]!;
-      }
+      var source = _declarationSources[declaration];
+      if (source != null) _sources[node] = source;
     } else if (namespace == null) {
       _unresolvedReferences[node] = _scope;
     }

pubspec.yaml

@@ -1,5 +1,5 @@
 name: sass_migrator
-version: 1.7.1
+version: 1.7.2
 description: A tool for running migrations on Sass files
 homepage: https://github.com/sass/migrator