Fix a case of "background thread updating UI" (#2916)

Author: dmoagx

Source/SPExtendedTableInfo.m

@@ -437,6 +437,8 @@ - (void)loadTable:(NSString *)table
 
 /**
  * Returns a dictionary describing the information of the table to be used for printing purposes.
+ *
+ * MUST BE CALLED ON THE UI THREAD!
  */
 - (NSDictionary *)tableInformationForPrinting
 {

Source/SPPrintController.m

@@ -364,7 +364,7 @@ - (void)generateTableInfoHTMLForPrinting
 	NSString *heading = NSLocalizedString(@"Table Information", @"table information print heading");
 
 	[engine setObject:connection forKey:@"c"];
-	[engine setObject:[extendedTableInfoInstance tableInformationForPrinting] forKey:@"i"];
+	[engine setObject:[[extendedTableInfoInstance onMainThread] tableInformationForPrinting] forKey:@"i"];
 
 	[printData setObject:heading forKey:@"heading"];
 	[printData setObject:[[NSUnarchiver unarchiveObjectWithData:[prefs objectForKey:SPCustomQueryEditorFont]] fontName] forKey:@"font"];