apply_change_to_project: Raise on project reference without file.

lib/kintsugi/apply_change_to_project.rb

@@ -524,6 +524,12 @@ def add_subproject_reference(root_object, project_reference_change)
         find_file(root_object.project, project_reference_change["ProjectRef"],
                   file_filter: filter_subproject_without_project_references)
 
+      unless subproject_reference
+        raise MergeError, "No file reference was found for project reference with change " \
+          "#{project_reference_change}. This might mean that the file used to exist in the " \
+          "project the but was removed at some point"
+      end
+
       attribute =
         Xcodeproj::Project::PBXProject.references_by_keys_attributes
                                       .find { |attrb| attrb.name == :project_references }

spec/kintsugi_apply_change_to_project_spec.rb

@@ -7,6 +7,7 @@
 require "tempfile"
 require "tmpdir"
 
+require "kintsugi/error"
 require "kintsugi/apply_change_to_project"
 
 require_relative "be_equivalent_to_project"
@@ -114,6 +115,23 @@
     expect(base_project).to be_equivalent_to_project(theirs_project, ignore_keys: ["containerPortal"])
   end
 
+  it "raises if adding subproject whose file reference isn't found" do
+    ours_project = create_copy_of_project(base_project.path, "ours")
+
+    add_new_subproject_to_project(base_project, "foo", "foo")
+    base_project.save
+
+    theirs_project = create_copy_of_project(base_project.path, "theirs")
+
+    base_project.root_object.project_references.pop
+
+    changes_to_apply = get_diff(theirs_project, base_project)
+
+    expect {
+      described_class.apply_change_to_project(ours_project, changes_to_apply)
+    }.to raise_error(Kintsugi::MergeError)
+  end
+
   describe "file related changes" do
     let(:filepath) { "foo" }