Fix an error for Layout/MultilineMethodCallIndentation with safe navigation and assignment method

changelog/fix_and_error_for_layout_multiline_method_call_indentation.md

@@ -0,0 +1 @@
+* [#12765](https://github.com/rubocop/rubocop/pull/12765): Fix an error for `Layout/MultilineMethodCallIndentation` with safe navigation and assignment method. ([@earlopain][])

lib/rubocop/cop/mixin/multiline_expression_indentation.rb

@@ -189,7 +189,7 @@ def assignment_rhs(node)
         case node.type
         when :casgn   then _scope, _lhs, rhs = *node
         when :op_asgn then _lhs, _op, rhs = *node
-        when :send    then rhs = node.last_argument
+        when :send, :csend then rhs = node.last_argument
         else               _lhs, rhs = *node
         end
         rhs

spec/rubocop/cop/layout/multiline_method_call_indentation_spec.rb

@@ -355,6 +355,19 @@
             )
         RUBY
       end
+
+      it "doesn't crash on multiline method calls with safe navigation and assignment" do
+        expect_offense(<<~RUBY)
+          MyClass.
+          foo&.bar = 'baz'
+          ^^^ Use 2 (not 0) spaces for indenting an expression spanning multiple lines.
+        RUBY
+
+        expect_correction(<<~RUBY)
+          MyClass.
+            foo&.bar = 'baz'
+        RUBY
+      end
     end
   end