Tabs cache set to "null" when using "add" function

$_getIdFromClassName is assigned "Null" to make it compatible with function "save". In P.S. 1.6.0.3 $_getIdFromClassName was assigned null after using "save function", but if a module is using "add" function new value is added to the array, but not reloaded. 

When a tab is created via "save" function $_getIdFromClassName is assigned "null", but if a module creates a tab using "add" function, newly adde tab is assign to $_getIdFromClassName (without reloading data from database), this causes $_getIdFromClassName to contain 1 value, and this prevents function "getIdFromClassName" to reload tabs data from database, and causes "getIdFromClassName" to return "false" in future calls.
Merge pull request #1417 from erickturcios/1.6

// Tabs cache is reset on "add" function