Add NPE protection due to apache/maven#11552

Author: laeubi

org.eclipse.m2e.core/src/org/eclipse/m2e/core/internal/embedder/MavenProjectMutableState.java

@@ -75,6 +75,8 @@ public void restore(MavenProject project) {
         project.getProperties().putAll(properties);
       } catch(UnsupportedOperationException e) {
         //if the collection itself is immutable then we do not need to restore a snapshot because it can not be altered anyways!
+      } catch(NullPointerException e) {
+        //see https://github.com/apache/maven/issues/11552
       }
     }