NullPointerException: Attempt to invoke virtual method on EditText
What is a NullPointerException, and how do I fix it?
rent a car, rent out a car, hire a car English not my native language. I'm trying to understand what would be correct in British English.  If I want to use a car for some time. I can say, "I want to rent a car". But from my knowledge there may be a  misunderstanding. I can say the same words if I want to give a car to someone to use for some time and I want to receive payment.  So what would be correct and not misunderstanding?   1) I want to use a car for some time = "I want to hire a car" 2) I want to give a car to someone to use for some time = "rent out a car"   Is my understanding correct? Flattening a shallow list in Python Is there a simple way to flatten a list of iterables with a list comprehension, or failing that, what would you all consider to be the best way to flatten a shallow list like this, balancing performance and readability?  I tried to flatten such a list with a nested list comprehension, like this:  [image for image in menuitem for menuitem in list_of_menuitems]   But I get in trouble of the NameError variety there, because the name 'menuitem' is not defined.  After googling and looking around on Stack Overflow, I got the desired results with a reduce statement:  reduce(list.__add__, map(lambda x: list(x), list_of_menuitems))   But this method is fairly unreadable because I need that list(x) call there because x is a Django QuerySet object.  Conclusion:   Thanks to everyone who contributed to this question.  Here is a summary of what I learned.  I'm also making this a community wiki in case others want to add to or correct these observations.  My original reduce statement is redundant and is better written this way:  &gt;&gt;&gt; reduce(list.__add__, (list(mi) for mi in list_of_menuitems))   This is the correct syntax for a nested list comprehension (Brilliant summary !):  &gt;&gt;&gt; [image for mi in list_of_menuitems for image in mi]   But neither of these methods are as efficient as using itertools.chain:  &gt;&gt;&gt; from itertools import chain &gt;&gt;&gt; list(chain(*list_of_menuitems))   And as @cdleary notes, it's probably better style to avoid * operator magic by using chain.from_iterable like so:  &gt;&gt;&gt; chain = itertools.chain.from_iterable([[1,2],[3],[5,89],[],[6]]) &gt;&gt;&gt; print(list(chain)) &gt;&gt;&gt; [1, 2, 3, 5, 89, 6] sstatic.net SSL certificate is for an incorrect domain How do I construct the $SU(2)$ representation of the Lorentz Group using $SU(2)\times SU(2)\sim SO(3,1)$ ? Use of “f ” instead of “s” in historic, printed English documents I was at a museum in London yesterday, and one of the items on exhibit is a document from the eighteenth century. It uses the letter f a lot where s should be used—for example, in Majefty.     Did the letter f once have the same sound as s today, or were some of those words actually spoken with an f ? Reading from text file until EOF repeats last line Does $\mathbb R^2$ remain connected when countably many points are removed? Does it remain path connected?  This is not homework but is in response to working several problems where countable or uncountable many points are removed and the pattern always holds.  Thank you. Calibre No Longer Working Calibre stopped working recently. Tried uninstalling and reinstalling. When I open a terminal and call $calibre I get the following error message: Traceback (most recent call last):   File &quot;/usr/bin/calibre&quot;, line 20, in &lt;module&gt;     sys.exit(calibre())   File &quot;/usr/lib/calibre/calibre/gui_launch.py&quot;, line 73, in calibre     main(args)   File &quot;/usr/lib/calibre/calibre/gui2/main.py&quot;, line 543, in main     listener = create_listener()   File &quot;/usr/lib/calibre/calibre/gui2/main.py&quot;, line 514, in create_listener     return Listener(address=gui_socket_address())   File &quot;/usr/lib/calibre/calibre/utils/ipc/server.py&quot;, line 110, in __init__     self._listener._unlink.cancel() AttributeError: 'NoneType' object has no attribute 'cancel'  Do I need to install some dependency that the sudo apt install didn't get? I tried installing via command line and the software installer/updater GUI. I'm on version Ubuntu 20.04.1 LTS. I made sure that my software in general is up to date also. ASP.NET Website's BIN directory and references How to check type of files without extensions in python? If you create a scene for the Video Sequence Editor and start working on a project Blender will render the output from the VSE. But if you want to later switch it to render your 3D scene again (the camera output) where is the setting to select the source?  Usually I keep a separate .blend for VSE edits so I don't need to switch, but I feel like I ought to learn how to do this as it's pretty basic. I would expect to find this in the Render context but I see no place to specify the source.  Thanks. Fiction novel or movie about sentient dinosaurs Can you find shadow orbs in terraria demo? I have the PS vita demo, and was wondering if you could find any shadow orbs in that tutorial world. Thanks. Prove the following: If $a \mid bc$, then $a \mid \gcd(a, b)c$. Prove the following: If $a \mid bc$, then $a  \mid \gcd(a, b)c$.   I tried to set $\gcd(a, b)$ to $b$ and used the fundamental theorem of arithmetic to prove that it is divisible by $a$, but I can't prove that $a \mid bc$, if and only if $a\mid b$ and $a\mid c$. Please help. Thanks. Proving a direct product of groups is a group Are questions about (development) devices allowed on Stack Overflow? I've seen many questions on Stack Overflow asking which devices are best for developers. This made me wonder if questions asking for hardware device recommendations (best device for programming, best device within a certain budget, etc.) are on-topic for Stack Overflow, or if they should be asked somewhere else?  For Instance check these links:         etc. Why is it true that "I before E, except after C"? Why use gradient descent with neural networks? When training a neural network using the back-propagation algorithm, the gradient descent method is used to determine the weight updates. My question is: Rather than using gradient descent method to slowly locate the minimum point with respect to a certain weight, why don't we just set the derivative $\frac{d(\text{Error})}{dw}=0$, and find the value of weight $w$ which minimizes the error? Also, why are we sure that the error function in back-propagation will be a minimum? Can't it turn out the error function is a maximum instead? Is there a specific property of the squashing functions that guarantees that a network with any number of hidden nodes with arbitrary weights and input vectors will always give an error function that has some minima? Any hints or answers to figure out the unknown value of C? Down Arrow Key is not working