@Mayojar77
 
Lots of good reasons:  
-Twi is unknown to the population of Equestria in the Crystal Empire. it was nothing offical just a call from her mentor.  
-She´s in the middle of a castle, surrounded by wall and patrolling guards.  
-She´s sleeping next to the rooms of her friends and the three other alicorn princesses are sleeping in the same castle as well.  
Ergo, no normal thief could have entered this room and steal the crown.  
Prob, but that´s Celestia´s fault really, Sunet Shimmer got into the castle via the teleport mirror and got past the guards with her shadow spell (at least it looked like she just became a shadow on the wall) though seriously those guards SUCKS at patrolling. Like a K.I. in a stealth game. XD
 
So yeah, none of that´s Twi´s fault, there were plently of reasons for her to believe that the crown´s safe on the nightstand.
 
@Mayojar77
 
I think you´re leaning to far outta your window here. For all we know the changelings DONT have a nation. Given the comic, Chrysalis and her changelings are more of a swarm which wanders through the lands to get their love to feed upon than have a steady realm. You could say they got a hive but then again we cant be sure of that given that she and her changelings never showed up after being thrown outta Equestria.   What we can say for sure given the comic and if the comic´s canon is that they DONT have a nation.
 
Glad to be of help to ya.^^

Aah just noticed the scrunchy face. How cute :)

@PonyPon  
I know right? What an absolute tragedy that would have been! /le sarcasm.

@Trickquestion  
Considering Equestria is kind of at war with a nation that as a rule specializes in espionage, that’s no excuse.

@Trickquestion  
Oh yeah. Wouldn’t want to keep this masterpiece of cinema from the big screen, would we?!

@Mayojar77
 
@PonyPon
 
@Sjogre  
She’s got a very good reason for the lack of security guys!
 
The movie wouldn’t have happened otherwise!

@Background Pony  
On their stomaches. Like horses.

@Mayojar77  
Or a modified version of the vault it was kept in before the third season.
 
Or, heck, the hidden chamber that King Sombra used.

@PonyPon  
Or in her indestructible bunker only she and Pinkie can open.

@Mayojar77  
Yeah, you’d think she would at least keep in a magical screaming chest or something.

Why did she put it on her fucking nightstand? It’s one of the most powerful magical artifacts, not some trinket.

For the last time, your wings are not accordions, Twilight.

She probably now wonders how pegasi sleep.