Prevent frozen string errors in promo screenshots action (#653)

Author: mokagio

CHANGELOG.md

@@ -14,7 +14,7 @@ _None_
 
 ### Bug Fixes
 
-_None_
+- Fix `can't modify frozen String: "transparent"` and other `FrozenError` crashes in `promo_screenshots_action`. [#653]
 
 ### Internal Changes
 

lib/fastlane/plugin/wpmreleasetoolkit/helper/promo_screenshots_helper.rb

@@ -24,10 +24,12 @@ module Helper
     class PromoScreenshots
       def initialize
         if $skip_magick
-          message = "PromoScreenshots feature is currently disabled.\n"
-          message << "Please, install RMagick if you aim to generate the PromoScreenshots.\n"
-          message << "'bundle install --with screenshots' should do it if your project is configured for PromoScreenshots.\n"
-          message << 'Aborting.'
+          message = <<~MSG
+            PromoScreenshots feature is currently disabled.
+            Please, install RMagick if you aim to generate the PromoScreenshots.
+            'bundle install --with screenshots' should do it if your project is configured for PromoScreenshots.
+            Aborting.
+          MSG
           UI.user_error!(message)
         end
 
@@ -400,10 +402,13 @@ def open_image(path)
       end
 
       def create_image(width, height, background = 'transparent')
-        background.paint.to_hex
+        # The paint method we call below modifies the string in place.
+        # But if the string is frozen, we need to dup it first, otherwise we'll get a frozen string error.
+        working_background = background.frozen? ? background.dup : background
+        working_background.paint.to_hex
 
         Image.new(width, height) do
-          self.background_color = background
+          self.background_color = working_background
         end
       end