Guard against restore being called with no previously saved state

lib/dotenv.rb

@@ -74,6 +74,9 @@ def save
   # @param env [Hash] Hash of keys and values to restore, defaults to the last saved state
   # @param safe [Boolean] Is it safe to modify `ENV`? Defaults to `true` in the main thread, otherwise raises an error.
   def restore(env = @diff&.a, safe: Thread.current == Thread.main)
+    # No previously saved or provided state to restore
+    return unless env
+
     diff = Dotenv::Diff.new(b: env)
     return unless diff.any?
 

spec/dotenv_spec.rb

@@ -325,6 +325,17 @@
       end.join
       expect(ENV["MODIFIED"]).to eq("true")
     end
+
+    it "is a noop if nil state provided" do
+      expect { Dotenv.restore(nil) }.not_to raise_error
+    end
+
+    it "is a noop if no previously saved state" do
+      # Clear state saved in setup
+      expect(Dotenv.instance_variable_get(:@diff)).to be_instance_of(Dotenv::Diff)
+      Dotenv.instance_variable_set(:@diff, nil)
+      expect { Dotenv.restore }.not_to raise_error
+    end
   end
 
   describe "modify" do