Prevent NPE when trying to set statement = null in SQLManager

If the statement is null, the `isClosed` check will lead to a NPE. As it's going to be null either way, just ignoring the NPE has no side effects.

Core/src/main/java/com/github/intellectualsites/plotsquared/plot/database/SQLManager.java

@@ -375,7 +375,7 @@ import java.util.concurrent.atomic.AtomicInteger;
                                 if (statement.isClosed()) {
                                     statement = null;
                                 }
-                            } catch (AbstractMethodError ignore) {
+                            } catch (NullPointerException | AbstractMethodError ignore) {
                             }
                         }
                         lastTask = task;