Seg fault for line fscanf(f,"%[^\n]%*c",temp);

Question

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int GET (FILE* f, char act[100],int check)
{
    if(check==0)
    {
        return 0;
        
    }
    
    char temp[20];
    fscanf(f,"%[^\n]%*c",temp);
    
    
    
    if(feof(f))
    {
        strcpy(act,NULL);
        return 0;
    }
    
    strcpy(act,temp);
    act[21]='\0';
    return 0;

}
int main(int argc, char** argv)
{
    FILE* fa = fopen(argv[1],"r");
    FILE* fb = fopen(argv[2],"r");

    FILE* f1 = fopen(argv[3],"w");
    char act1[100];
    char act2[100];
    int flagA=1;
    int flagB=1;

    
   
    
    while(1)
    {
        GET(fa,act1,flagA);
        GET(fb,act2,flagB);
        if(strcmp(act1,NULL)==0 && strcmp(act1,NULL)==0)
        {
            break;
        }
        int a = strcmp(act1,act2);
        if(a==0)
        {
            flagA=1;
            flagB =1;
            fprintf(f1,"%s\n%s\n",act1,act2);
        }
        else if(a>0)
        {
            flagA=0;
            flagB =1;
            fprintf(f1,"%s\n",act2);
        }
        else
        {
            flagA=1;
            flagB =0;
            fprintf(f1,"%s\n",act1);
        }
        
    }
    
    
    fclose(fa);
    fclose(fb);
    fclose(f1);
    
    return 0;
}

The program is supposed to take two files each containing a sorted list of names and combine them into one file, with the names sorted. I'm getting a segmentation fault on the line

fscanf(f,"%[^\n]%*c",temp); It worked perfectly when I ran a much simpler program to take names from a file and print it to another file. The file format is Name1 Name2 Name3 . . . So on. Each name is not more than 15 characters long.

Any help would be greatly appreciated. Thank You.