How To Fix: Modifying requirements before initialization is not supported

This is an easy fix as long as you understand what the error is telling you.

TLDR make sure you always initialize your queries in your processor constructor.

I sometimes see this error when I add a new query to an existing processor and I forget to update the constructor. Your mileage may vary.

Solution: Initialize your Query Constructor in your Processor Constructor

USampleProcessor::USampleProcessor()
    : Query(*this)  // <-- Add this line
{
    ExecutionFlags = static_cast<int32>(EProcessorExecutionFlags::All);
}

Determine which processor is the problem

In the crash callstack, check which processor’s ConfigureQueries method is being executed. This is the problematic processor whose constructor needs to initialize the query, in the below example it is USampleProcessor.

FMassFragmentRequirements::AddRequirement<…>(EMassFragmentAccess, EMassFragmentPresence) MassRequirements.h:235
USampleProcessor::ConfigureQueries(const TSharedRef<…> &) USampleProcessor.cpp:242
UMassProcessor::CallInitialize(TNotNull<…>, const TSharedRef<…> &) MassProcessor.cpp:170
# snip lots of other callstack below here

Problem: Crash with this error

If you crash to this line in MassRequirements.h you’re experiencing this error.

template<typename T>
FMassFragmentRequirements& AddRequirement(const EMassFragmentAccess AccessMode, const EMassFragmentPresence Presence = EMassFragmentPresence::All)
{
    checkf(bInitialized, TEXT("Modifying requirements before initialization is not supported."));

    // [xist] The above line is where it crashes. Removed code below it for brevity.
}