@anneellis11 also i have had the same problem on different installations. The theme has nothing to do with the problem that resulted from the plugin. To fix it open the file
/yourWordpressPath/wp-content/plugins/ultimate-post/Initialization.php
Change line 517 from this
public function block_return ($blocks, $paged, $blockId, $blockRaw, $blockName, $builder) {
to so
public function block_return ($blocks, $paged, $blockId, $blockRaw, $blockName, $builder = null) {