Buggy python submission (verdict: Runtime Error):
import numpy as np
from collections import deque
infinity = -10**5
H,W = map(int,input().split())

A = np.zeros((H+1,W+1))

for i in range(H):
    s = input()
    for j in range(W):
        if s[j] == ".":
            A[i][j] = 1
        else:
            A[i][j] = -1

N = H*W
G=[[] for i in range(N)]

for i in range(H):
    for j in range(W):
        if A[i][j] ==1:
            if A[i+1][j] == 1:
                G[W*(i+1)+j].append(W*i+j)
                G[W*i+j].append(W*(i+1)+j)

            if A[i][j+1] == 1:
                G[W*i+(j+1)].append(W*i+j)
                G[W*i+j].append(W*i+(j+1))

def BFS (s):
    queue = deque()
    d = [infinity]*N
    
    queue.append(s)
    d[s]= 0
    
    while len(queue)!=0:
      u = queue.popleft()
      for v in G[u]:
      	if d[v] == infinity:
        	d[v] = d[u]+1
        	queue.append(v) 	  
  
    return d

ans = 0

for i in range(H):
    for j in range(W):
        if A[i][j] == 1:
            d = BFS(W*i+j)
        ans = max(ans,max(d))
print(ans)

    

Fixed submission:
import numpy as np
from collections import deque
infinity = 10**5
H,W = map(int,input().split())

A = np.zeros((H+1,W+1))

for i in range(H):
    s = input()
    for j in range(W):
        if s[j] == ".":
            A[i][j] = 1
        else:
            A[i][j] = -1
#print(A)
N = H*W
G=[[] for i in range(N)]

for i in range(H):
    for j in range(W):
        if A[i+1][j] == 1:
            #if W*(i+1)+j ==1:
                #print("a")
            #G[W*(i+1)+j].append(W*i+j)
            G[W*i+j].append(W*(i+1)+j)

        if i>=1:
            if A[i-1][j] == 1:
                #if W*(i-1)+j ==1:
                    #print("b")
                #G[W*(i-1)+j].append(W*i+j)
                G[W*i+j].append(W*(i-1)+j)            
            
        
        if A[i][j+1] == 1:
            #if W* ==1:
                #print("c")
            #G[W*i+(j+1)].append(W*i+j)
            G[W*i+j].append(W*i+(j+1))
        
        if j>=1:
            if A[i][j-1] == 1:
                #if W*(i+1)+j ==1:
                    #print("d")
                #G[W*i+(j-1)].append(W*i+j)
                G[W*i+j].append(W*i+(j-1))

def BFS (s):
    queue = deque()
    d = [infinity]*N
    
    queue.append(s)
    d[s]= 0
    
    while len(queue)!=0:
      u = queue.popleft()
      for v in G[u]:
      	if d[v] == infinity:
        	d[v] = d[u]+1
        	queue.append(v) 	  
  
    return d

ans = 0

#print(G)

for i in range(N):
    #print(i)
    I = i//W
    J = i%W
    #print(I,J)
    if A[I][J] == 1:
        d = BFS(i)
        for k in range(N):
            if d[k] == infinity:
                d[k] = -1
        ans = max(ans,max(d))

print(ans)

Buggy python submission (verdict: Runtime Error):
import numpy as np
from collections import deque
infinity = 10**5
H,W = map(int,input().split())

A = np.zeros((H+1,W+1))

for i in range(H):
    s = input()
    for j in range(W):
        if s[j] == ".":
            A[i][j] = 1
        else:
            A[i][j] = -1

N = H*W
G=[[] for i in range(N)]

for i in range(H):
    for j in range(W):
        if A[i+1][j] == 1:
            G[W*(i+1)+j].append(W*i+j)
            #G[W*i+j].append(W*(i+1)+j)

        if i>=1:
            if A[i-1][j] == 1:
                G[W*(i-1)+j].append(W*i+j)
                #G[W*i+j].append(W*(i-1)+j)            
            
        
        if A[i][j+1] == 1:
            G[W*i+(j+1)].append(W*i+j)
            #G[W*i+j].append(W*i+(j+1))
        
        if j>=1:
            if A[i][j-1] == 1:
                G[W*i+(j-1)].append(W*i+j)
                #G[W*i+j].append(W*i+(j-1))

def BFS (s):
    queue = deque()
    d = [infinity]*N
    
    queue.append(s)
    d[s]= 0
    
    while len(queue)!=0:
      u = queue.popleft()
      for v in G[u]:
      	if d[v] == infinity:
        	d[v] = d[u]+1
        	queue.append(v) 	  
  
    return d

ans = 0
#print(G)
for i in range(N):
    #print(i)
   
    I = i//W
    J = i%W
    #print(I,J)
    if A[I][J] == 1:
        d = BFS(i)
        for k in range(N):
            if d[k] == infinity:
                d[k] = -1
        ans = max(ans,max(d))

print(ans)

Fixed submission:
import numpy as np
from collections import deque
infinity = 10**5
H,W = map(int,input().split())

A = np.zeros((H+1,W+1))

for i in range(H):
    s = input()
    for j in range(W):
        if s[j] == ".":
            A[i][j] = 1
        else:
            A[i][j] = -1
#print(A)
N = H*W
G=[[] for i in range(N)]

for i in range(H):
    for j in range(W):
        if A[i+1][j] == 1:
            #if W*(i+1)+j ==1:
                #print("a")
            #G[W*(i+1)+j].append(W*i+j)
            G[W*i+j].append(W*(i+1)+j)

        if i>=1:
            if A[i-1][j] == 1:
                #if W*(i-1)+j ==1:
                    #print("b")
                #G[W*(i-1)+j].append(W*i+j)
                G[W*i+j].append(W*(i-1)+j)            
            
        
        if A[i][j+1] == 1:
            #if W* ==1:
                #print("c")
            #G[W*i+(j+1)].append(W*i+j)
            G[W*i+j].append(W*i+(j+1))
        
        if j>=1:
            if A[i][j-1] == 1:
                #if W*(i+1)+j ==1:
                    #print("d")
                #G[W*i+(j-1)].append(W*i+j)
                G[W*i+j].append(W*i+(j-1))

def BFS (s):
    queue = deque()
    d = [infinity]*N
    
    queue.append(s)
    d[s]= 0
    
    while len(queue)!=0:
      u = queue.popleft()
      for v in G[u]:
      	if d[v] == infinity:
        	d[v] = d[u]+1
        	queue.append(v) 	  
  
    return d

ans = 0

#print(G)

for i in range(N):
    #print(i)
    I = i//W
    J = i%W
    #print(I,J)
    if A[I][J] == 1:
        d = BFS(i)
        for k in range(N):
            if d[k] == infinity:
                d[k] = -1
        ans = max(ans,max(d))

print(ans)