fix missing SloeberProject_Project_is_null message

Author: jan

io.sloeber.core/src/io/sloeber/core/api/messages.properties

@@ -91,4 +91,4 @@ CompileDescription_WarningsCustom=Custom
 CompileDescription_WarningsDefault=Default
 CompileDescription_WarningsMore=More
 CompileDescription_WarningsNone=None
-
+SloeberProject_Project_is_null=The provided project is null. Sloeber can not upgrade.