Fix modproperties property in mods.toml causing exception (#7192)

src/fmllauncher/java/net/minecraftforge/fml/loading/moddiscovery/ModFileInfo.java

@@ -70,8 +70,7 @@ public class ModFileInfo implements IModFileInfo, IConfigurable
         this.license = config.<String>getConfigElement("license")
             .orElseThrow(()->new InvalidModFileException("Missing License, please supply a license.", this));
         this.showAsResourcePack = config.<Boolean>getConfigElement("showAsResourcePack").orElse(false);
-        this.properties = config.<UnmodifiableConfig>getConfigElement("properties").
-                map(UnmodifiableConfig::valueMap).orElse(Collections.emptyMap());
+        this.properties = config.<Map<String, Object>>getConfigElement("properties").orElse(Collections.emptyMap());
         this.modFile.setFileProperties(this.properties);
         this.issueURL = config.<String>getConfigElement("issueTrackerURL").map(StringUtils::toURL).orElse(null);
         final List<? extends IConfigurable> modConfigs = config.getConfigList("mods");

src/fmllauncher/java/net/minecraftforge/fml/loading/moddiscovery/NightConfigWrapper.java

@@ -45,8 +45,14 @@ public class NightConfigWrapper implements IConfigurable {
     }
 
     @Override
+    @SuppressWarnings("unchecked")
     public <T> Optional<T> getConfigElement(final String... key) {
-        return this.config.getOptional(asList(key));
+        return this.config.getOptional(asList(key)).map(value -> {
+            if (value instanceof UnmodifiableConfig) {
+                return (T) ((UnmodifiableConfig) value).valueMap();
+            }
+            return (T) value;
+        });
     }
 
     @Override